Removes unnecessary MenuButton styles and workarounds and fixes broken states of Animation button as a result.

MenuButton doesn't need any styles, it has flat = true by default (disables all styleboxes) and when it's false godot consistently uses FlatMenuButton type variation everywhere to make them flat but with hover/pressed states. There are only 3 non-flat menu buttons in the entire editor with both flat = false and no FlatMenuButton on them (Animation button, track Edit button and Actions in import dialog), all of which have overrides in _notification to make them look like regular Button.

This PR makes MenuButton inherit Button styles directly to remove those band-aids and match the expectation of it being a regular Button as opposed to FlatMenuButton. I went through all of the MenuButtons in editor to make sure they consistently use FlatMenuButton type variation where they should be flat, there are only a few such cases where it was missing.

Classic theme, pressed state had wrong stylebox:

Master	PR

Modern theme, hovered state had wrong stylebox:

Master	PR