Reorganized Git menu

Suggestion for reorganized Git menu,
because there are a lot of items in there

Task-number: QTCREATORBUG-8363

Change-Id: I00bcf6b210d9a2bcb081a17a0ea288d71f65a9f4
Reviewed-by: Orgad Shaneh <orgads@gmail.com>
Reviewed-by: Petar Perisin <petar.perisin@gmail.com>
Reviewed-by: Eike Ziller <eike.ziller@digia.com>
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>