QmlProfiler: Make sure TimelineRenderer is using the right x and width

If we use different x and width parameters in JavaScript and C++ we
introduce numerical instability which leads to visual glitches.

Change-Id: I352f3e8365ca52d135230343c2f034332ec71323
Task-number: QTCREATORBUG-12648
Reviewed-by: Kai Koehne <kai.koehne@digia.com>
QmlProfiler: Keep TimelineRenderer and Button states in sync

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>