Fix selection box + scrolling in animation editor.

[SaracenOne]

Closes #76819

godot.windows.editor.dev.x86_64_aCj3kuXTnJ.mp4

Attempts to fix this by introducing the following changes:

• Disable the panning behaviour when the selection box is active.
• Places the selection box in a clipped container used for the timeline.
• Dynamically repositions the selection box in response to changes to the horizontal and vertical scrollers.

In theory, it should also work with horizontal scrolling too, but I don't have access to a mouse which can do this. If anyone has access to one (I think most Mac computers have a mouse which can do this), please confirm that this fix also works for horizontal scrolling.

[KoBeWi]

In theory, it should also work with horizontal scrolling too, but I don't have access to a mouse which can do this.

You can scroll horizontally by holding Shift.

EDIT:
For some reason it doesn't work when dragging 🤔

[SaracenOne]

Updated to fix the issues raised. Regarding the Shift+Mouse scroll thing, I believe this is a feature of the ViewPanner which is attached to the Timeline scrollbox, which also gives other navigation controls like being able to scroll through the timeline by clicking and dragging with the middle mouse button. I modified the behaviour to intentionally 'not' call the panner while the selection box is active due to certain undesired behaviour in the site.

Perhaps it might be worth keeping the panner active instead, but modifying its functionality when the selection box is active to disable certain features. Regardless, I still think we need testing with omnidirectional mouse scrolling, but I think it would also still be okay to merge as is since its definitely an improvement on the old behaviour in any capacity.

[akien-mga]

Thanks!