Add selection box movement/scaling to the animation bezier editor

[YeldhamDev]

Implements godotengine/godot-proposals#3532 for the bezier editor.

Screencast_20241216_114037.webm

Sponsored By: 🐺 Lone Wolf Technology / 🍀 W4 Games.

[Arnklit]

Thanks so much for this @YeldhamDev , this will be such an improvement to the Bezier editor. I've been trying it out a bit and I did see some weird behaviour when selecting some of the bezier handles after doing a multi selection.

godot.windows.editor.dev.x86_64_0dLHJal9pr.mp4

[YeldhamDev]

@Arnklit This appears to be a preexisting bug unrelated to the PR (it also happens on 4.3). From what I tested, it seems to happen because the handle's position is being calculated relatively to the highlighted track.

[YeldhamDev]

It was a single line of code using the wrong variables. Added the fix in here too, why not.

[Arnklit]

It was a single line of code using the wrong variables. Added the fix in here too, why not.

Sorry I did not realise that was a pre-existing issue, thanks for including the fix. Tested again and yeah it's great now.

[Arnklit]

I had a go at adding in the cursor changing based on what handle you are dragging. Don't know if you want to incorporate that, otherwise I can do a PR for it when this one merges :). Arnklit@071f462

godot.windows.editor.dev.x86_64_WT7XvpX4gk.mp4

[YeldhamDev]

@Arnklit I think it's probably better to be its own separate PR.