Implement new_event_loop_proxy for PluginCanvasPlatform

[emuell]

This is needed for slint::invoke_from_event_loop and similar functions to work.

I started using invoke_from_event_loop and wondered why it didn't work. The missing impl can be worked around by adding a custom thread-safe event queue in custom editors, but it's nice to use the well-documented Slint feature instead.

[ilmai]

Hey, thanks for the PRs. Just FYI I'm quite busy with my plugin release right now so might take a while to review especially the larger standalone build PR.

[emuell]

Sure thing. Nothing's urgent here. Good luck with the release!

[emuell]

I don't think the order is important here, and also not which window actually pulls and applies the event. Main purpose of slint::invoke_from_event_loop() is to push an event to the event thread.

If an event references a window, it must pass a weak ref to the window anyway and check if it's still valid. Just like https://docs.rs/slint/latest/slint/struct.Weak.html#method.upgrade_in_event_loop does.

Also I don't see how to access PluginCanvasWindowAdapter within new_event_loop_proxy. Would need to do some ugly thread_local hacking to pass around the context then.

[ilmai]

Good point, this is similar to the slint limitation which made me use thread locals when generating window adapters (which is fragily in theory but probably works in practice). We can keep this as is. Maybe add a comment explaining that this is making plugin instances share the event queue which isn't ideal but current slint limitations don't allow us to separate them.