PluginManager: Clean up interface a bit

Change-Id: I6ce4443115c359c2c9588ffa615d13c29c249df2
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>
Add a toolbutton for switching diff view mode

Task-number: QTCREATORBUG-10035

Change-Id: I83cb7cec3f4ba484ab068f398a12096501017668
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>