On Wayland, reduce amount of spurious wakeups

Mark it as breaking, since some clients relied on that behavior, simply
because dispatching clients queue always woke up a winit, meaning that
they won't be able to use user events for this sake.

src/platform_impl/linux/wayland/event_loop/sink.rs

@@ -20,6 +20,12 @@ impl EventSink {
         Default::default()
     }
 
+    /// Return `true` if there're pending events.
+    #[inline]
+    pub fn is_empty(&self) -> bool {
+        self.window_events.is_empty()
+    }
+
     /// Add new device event to a queue.
     #[inline]
     pub fn push_device_event(&mut self, event: DeviceEvent, device_id: DeviceId) {