Pull all version control UI from Godot into the Git plugin

We experimented with adding UI nodes to the editor from GDNative earlier and likely it should be possible with GDExtension as well.

This gives us good control over the UI for the Git integration because it would completely decouple Godot's UI source code from the Git plugin, except the dependency on the EditorVCSInterface, which seems inevitable.

This would also mean we would enable VCS plugins to show customized UIs for different VCSs instead of us trying to abstract them away completely. Overly generalized UIs can lead to a worse user experience.