Events rework

[almarklein]

I plan to combine some work on events, probably merging multiple PRs in a separate branch and then merging in main when done. Then do a major version bump, because there will be backwards incompatible changes.

My plan is to introduce a new method, e.g. canvas.connect_event or canvas.events.connect(), so that we can keep most backwards incompatible changes scoped to that new method, and leave canvas.add_event_handler() mostly (but not completely) unaffected.

Topics to address:

• Weak refs for event handlers? #240 (backwards incompatible wrt potentially dropping refs)
• Typed events #220 (backwards incompatible if we chose dataclasses.
• Renaming some events for better structure w.r.t. typing. (obviously backwards incompatible).
• Allow event-consuming code to set event['request_draw'] #189
• More consistent events #185
• Inconsistent modifiers on MacOS #154
• Figure out how to handle unicode char events #28
• Pinch and other Gestures #31

For canvas.add_event_handler(), only the subtle changes in #185 should be of concern. This relates to e.g. whether a new pointer_down is emitted when a pointer button is already down.

Event work for later:

• Animation events #239 (probably after adopting new style events in pygfx)
• More of Allow event-consuming code to set event['request_draw'] #189
• Make it possible to emulate events in an offscreen canvas #54