Add EventLoopBuilder

This commit adds an `EventLoopBuilder` struct to simplify event loop
customization and providing options to it upon creation. It also
deprecates the use of `EventLoop::with_user_event` in favor of the same
method on new builder, and replaces old platforms specific extension
traits with the new ones on the `EventLoopBuilder`.

src/platform_impl/android/mod.rs

@@ -71,6 +71,9 @@ pub struct EventLoop<T: 'static> {
     running: bool,
 }
 
+#[derive(Default, Debug, Copy, Clone, PartialEq, Hash)]
+pub(crate) struct PlatformSpecificEventLoopAttributes {}
+
 macro_rules! call_event_handler {
     ( $event_handler:expr, $window_target:expr, $cf:expr, $event:expr ) => {{
         if let ControlFlow::ExitWithCode(code) = $cf {
@@ -82,7 +85,7 @@ macro_rules! call_event_handler {
 }
 
 impl<T: 'static> EventLoop<T> {
-    pub fn new() -> Self {
+    pub(crate) fn new(_: &PlatformSpecificEventLoopAttributes) -> Self {
         Self {
             window_target: event_loop::EventLoopWindowTarget {
                 p: EventLoopWindowTarget {