Stop place mode when switching to pose mode

See Issue #13923
Add Delete Armature Action to context menu

The delete armature action deletes the armature and all its bones. It only
appears when the user clicks an armature node.
It was also the occasion to do a little bit a cleaning in the code.
The two methods calles upon currentNodeChanged() were merged together and
the method named updateArmatureWidget renamed to the simple updateWidget.

See Issue #13923