[ComboTabBar] Correctly returns tabAt over tab icon
[GreaseMonkey] Fix coding style
[TabBar] Use contextMenuEvent instead of customContextMenu signals
[ComboTabBar] Correctly switch tabs when closing last tab in tabbar

Also remove custom logic for SelectPreviousTab on tab remove and use
only the QTabBar's selectionBehaviorOnRemove

Fix 1 for #1322
[TabBar] Show add tab button at correct position when there are no normal tabs

Fix 2 for #1322
Closes #1322