Refactor event processing in Lorien

[MrApplejuice]

Why is this needed?

Pretty much what was discussed in here: https://github.com/mbrlabs/Lorien/discussions/144#discussioncomment-2966218

I had to extend it a bit further however. Problem is that all tools and the Viewport itself also tried to process events in various ways that would not be compatible with stopping event processing through the standard _input/_gui_input/_unhandled_input functions. I tried to push everything to _unhandled_input, but was not able to, because of the way the ViewportContainer is structured at the moment.

To limit the extent of the PR, I pushed everything to use at least _gui_input which is enough to be intercepted in the _input method in case the keybinding-dialog wants to listen for key events in the future.

More explanations in the code comments...

What changed

• Rewrote most event handling to use event-callbacks
• This allows events to be intercepted in the _input method
• I tried to rewrite everything so that input events are passed in _unhandled_input functions, bit was not able to do so for the ViewportContainer. Had to fallback on _gui_input instead which is good enough for my purpose. _unhandled_input really would be more desirable but requires a larger restructuring and putting all tools IN the viewport (I think).
• There is one last use of _input but it does not handle events as the other old _event functions did: ColorPalettePicker still uses _input to decide when to close. It does not interfer with anything else though, so this should be okay.

[MrApplejuice]

Converted to a Draft temporarily, because I think I found a problem with deleting strokes... need to investigate this tomorrow. No time tonight :-/

[MrApplejuice]

Key inputs fixed. Stuff can be deleted/copied/pasted again... Sorry for (almost) breaking this :P

[mbrlabs]

Not a problem, thanks for doing this. This is much cleaner than the previous input handling.