Add EventsLoop::wakeup independent of interrupt

[jwilm]

The method EventsLoop::interrupt() is currently serving the dual purpose of

1. Stop processing events during run_forever and poll_events
2. Wakeup the event loop from other threads

Running the wakeup code when you only care about 1. simply wastes cycles.

I'm proposing that a second method, EventsLoop::wakeup() is added to allow performing these tasks independently. An alternative design would be removing the interrupted flag altogether and allow the closure passed to run_forever and poll_events to return a flag indicating their desire to continue or halt.

[jwilm]

Btw, I'd be happy to implement this. I thought discussion was an appropriate first step, though.

[jwilm]

I've thought about this a bit more, and I think it would be appropriate for the wakeup method to take a winit::WindowId so that the Awakened event is associated with the correct window.

[jwilm]

Digging into the source a bit, it looks like there's some leftover WindowProxy implementations. These aren't available in the public API anymore since the recent event handling refactor. Seems like they should be removed / be absorbed into EventsLoop::wakeup implementations.

[jwilm]

Does anyone have feedback about this?

[mitchmindtree]

@jwilm just thought I'd let you know that #136 and in particular this comment are related to (and probably supersede) this.

[jwilm]

Agreed that the pending Proxy type should supersede this.