Git: Fix issues in the branch dialog

* Split part/next/last into a 'part' node and a 'next/last' node
  instead of 'part/next' and 'last' node. This is closer to what git does.

* Fix some model-issues along the way;)

* Unhighlight remote checked-out branches again once they are no longer
  the current one.

* Remove stale nodes after removing a branch. When removing 'first/next'
  kind of branches, do not keep a empty 'first' in the tree.

Task-number: QTCREATORBUG-8518

Change-Id: Ia5650540aa58354aaab513199c8622392bdbd37f
Reviewed-by: Orgad Shaneh <orgads@gmail.com>
Reviewed-by: Petar Perisin <petar.perisin@gmail.com>