You could enable the select-on-hover mode by clicking in an empty time
line, even though the buttons bar was disabled. You could also trigger
various bad behavior by clearing the view and restarting the profiler.
Properly initializing and clearing all members, and sending change
signals on clearing helps with that.

Change-Id: Ie7df80a52daaf166c3a4adb2eb1f28a59c21df57
Reviewed-by: Kai Koehne <kai.koehne@digia.com>