Add useful signals to QTabBar and QTabWidget

In this patch we introduce tabBarClicked and tabBarDoubleClicked to get
a finer grained information on the user interaction with the tab bar.

Done-with: kevin.ottens@kdab.com

Change-Id: I7be76a556ca09186e98f2e076fe2512d6c5e6773
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@digia.com>