Event Loop 2.0 API and Windows implementation (#638)

* Rename EventsLoop and associated types to EventLoop

* Rename WindowEvent::Refresh to WindowEvent::Redraw

* Remove second thread from win32 backend

* Update run_forever to hijack thread

* Replace windows Mutex with parking_lot Mutex

* Implement new ControlFlow and associated events

* Add StartCause::Init support, timer example

* Add ability to send custom user events

* Fully invert windows control flow so win32 calls into winit's callback

* Add request_redraw

* Rename platform to platform_impl

* Rename os to platform, add Ext trait postfixes

* Add platform::desktop module with EventLoopExt::run_return

* Re-organize into module structure

* Improve documentation

* Small changes to examples

* Improve docs for run and run_return

* Change instances of "events_loop" to "event_loop"

* Rename MonitorId to MonitorHandle

* Add CHANGELOG entry

* Improve WaitUntil timer precision

* When SendEvent is called during event closure, buffer events

* Fix resize lag when waiting in some situations

* Update send test and errors that broke some examples/APIs

* Improve clarity/fix typos in docs

* Fix unreachable panic after setting ControlFlow to Poll during some RedrawRequested events.

* Fix crash when running in release mode

* Remove crossbeam dependency and make drop events work again

* Remove serde implementations from ControlFlow

* Fix 1.24.1 build

* Fix freeze when setting decorations

* Replace &EventLoop in callback with &EventLoopWindowTarget

* Document and implement Debug for EventLoopWindowTarget

* Fix some deadlocks that could occur when changing window state

* Fix thread executor not executing closure when called from non-loop thread

* Fix buffered events not getting dispatched

* Fix crash with runner refcell not getting dropped

* Address review feedback

* Fix CHANGELOG typo

* Catch panics in user callback

src/platform_impl/linux/x11/util/mod.rs

@@ -0,0 +1,104 @@
+// Welcome to the util module, where we try to keep you from shooting yourself in the foot.
+// *results may vary
+
+mod atom;
+mod client_msg;
+mod format;
+mod geometry;
+mod hint;
+mod icon;
+mod input;
+mod memory;
+mod randr;
+mod window_property;
+mod wm;
+
+pub use self::atom::*;
+pub use self::client_msg::*;
+pub use self::format::*;
+pub use self::geometry::*;
+pub use self::hint::*;
+pub use self::icon::*;
+pub use self::input::*;
+pub use self::memory::*;
+pub use self::randr::*;
+pub use self::window_property::*;
+pub use self::wm::*;
+
+use std::mem;
+use std::ptr;
+use std::ops::BitAnd;
+use std::os::raw::*;
+
+use super::{ffi, XConnection, XError};
+
+pub fn reinterpret<'a, A, B>(a: &'a A) -> &'a B {
+    let b_ptr = a as *const _ as *const B;
+    unsafe { &*b_ptr }
+}
+
+pub fn maybe_change<T: PartialEq>(field: &mut Option<T>, value: T) -> bool {
+    let wrapped = Some(value);
+    if *field != wrapped {
+        *field = wrapped;
+        true
+    } else {
+        false
+    }
+}
+
+pub fn has_flag<T>(bitset: T, flag: T) -> bool
+where T:
+    Copy + PartialEq + BitAnd<T, Output = T>
+{
+    bitset & flag == flag
+}
+
+#[must_use = "This request was made asynchronously, and is still in the output buffer. You must explicitly choose to either `.flush()` (empty the output buffer, sending the request now) or `.queue()` (wait to send the request, allowing you to continue to add more requests without additional round-trips). For more information, see the documentation for `util::flush_requests`."]
+pub struct Flusher<'a> {
+    xconn: &'a XConnection,
+}
+
+impl<'a> Flusher<'a> {
+    pub fn new(xconn: &'a XConnection) -> Self {
+        Flusher { xconn }
+    }
+
+    // "I want this request sent now!"
+    pub fn flush(self) -> Result<(), XError> {
+        self.xconn.flush_requests()
+    }
+
+    // "I want the response now too!"
+    pub fn sync(self) -> Result<(), XError> {
+        self.xconn.sync_with_server()
+    }
+
+    // "I'm aware that this request hasn't been sent, and I'm okay with waiting."
+    pub fn queue(self) {}
+}
+
+impl XConnection {
+    // This is impoartant, so pay attention!
+    // Xlib has an output buffer, and tries to hide the async nature of X from you.
+    // This buffer contains the requests you make, and is flushed under various circumstances:
+    // 1. `XPending`, `XNextEvent`, and `XWindowEvent` flush "as needed"
+    // 2. `XFlush` explicitly flushes
+    // 3. `XSync` flushes and blocks until all requests are responded to
+    // 4. Calls that have a return dependent on a response (i.e. `XGetWindowProperty`) sync internally.
+    //    When in doubt, check the X11 source; if a function calls `_XReply`, it flushes and waits.
+    // All util functions that abstract an async function will return a `Flusher`.
+    pub fn flush_requests(&self) -> Result<(), XError> {
+        unsafe { (self.xlib.XFlush)(self.display) };
+        //println!("XFlush");
+        // This isn't necessarily a useful time to check for errors (since our request hasn't
+        // necessarily been processed yet)
+        self.check_errors()
+    }
+
+    pub fn sync_with_server(&self) -> Result<(), XError> {
+        unsafe { (self.xlib.XSync)(self.display, ffi::False) };
+        //println!("XSync");
+        self.check_errors()
+    }
+}