Add a "Git branches" side panel

The Git branches dialog is nice, but often comes in my way when I need to move commits around, resolve merge conflicts, compile again, etc. It is in fact the dialog I have open most of the time working in Creator.

Given that we now also have a right side panel, it would be nice to have this function integrated there.

Maybe there could be an option "dock" and "undock" to switch between the two implementations, so the needs of all users are satisfied.