Windows: CursorState improvements (#526)

* Windows: CursorState improvements

Fixes #523

Prior to changing the cursor state, we now check the current grab
state, since it can be invalidated by alt-tabbing and other things.

`CursorState::Hide` is also implemented now.

The cursor name is now wrapped in a `Cursor` struct to allow
multithreaded access.

`Window::set_cursor_state` has been reworked to use
`execute_in_thread`. Two unneeded `transmute` calls were also
removed.

The `WM_SETCURSOR` handler is much more readable now.

`MonitorId::get_adapter_name` has been removed, since it's dead
code and appears to be a relic from 4 years ago.

* Windows: CursorState::Grab no longer hides cursor

`MouseCursor::NoneCursor` has been implemented to allow for
equivalent behavior to the older implementation.

Windows and X11 now have consistent cursor grabbing behavior.
macOS still needs to be updated.

* Windows: Grabbing auto-hides again (for now)

This API needs more work, so let's stick to a bug fix and some
refactoring. However, it now hides using a different technique
than it did originally, which applies instantly instead of after
mouse movement.

src/platform/windows/monitor.rs

@@ -152,13 +152,6 @@ impl MonitorId {
         self.dimensions
     }
 
-    /// This is a Win32-only function for `MonitorId` that returns the system name of the adapter
-    /// device.
-    #[inline]
-    pub fn get_adapter_name(&self) -> &[wchar_t] {
-        &self.adapter_name
-    }
-
     /// A window that is positioned at these coordinates will overlap the monitor.
     #[inline]
     pub fn get_position(&self) -> (i32, i32) {