Add theming support for hovered tabs

[davthedev]

This adds a distinct theming possibility for tabs when hovered by the mouse.

Proposal: https://github.com/godotengine/godot-proposals/issues/6462

Comes with presets for the editor & default theme generators.

This change is compatible with 3.x, but the theme generators must be adapted for the 3.x styles.

[MewPurPur]

Remember that your commits need to be squashed into one before the PR can be merged. Refer here: https://docs.godotengine.org/en/stable/community/contributing/pr_workflow.html#the-interactive-rebase

[akien-mga]

Thanks!