[WIP] Add box scaling to value tracks

[Arnklit]

This adds box scaling to the Value Tracks Editor, similar to what was added to the Bezier Track Editor
This would resolve godotengine/godot-proposals#3532

godot.windows.editor.dev.x86_64_fO9duTEQA2.mp4

• hide the lines if they are outside the range
• make the scale to offset by 12 pixels so it stays offset like the handle is
• make inverted scaling work
• make the handle offset correct when doing inverted scaling
• update the lines connecting identical keys when scaling
• adjust the rects so they are 5 pixels pushed away from the keys so it's easier to move the keys.
• key snapping
• snap the scaling handle?

This ended up being a bit of a bigger PR than I had hoped, so it needs a thorough review. I'll set it for draft now until I've had the chance to give it a thorough lookover myself.

[Arnklit]

I realise now that this might collide with the work in #105151

I had not realised someone else was working on this.

EDIT: Having a look at that PR, it looks like it was decided to go with an implementation more similar to what I have done here, so it might make sense to supercede.