C++: Only start/stop timers from the UI thread.

Change-Id: Ib694d2aeaef3b62caf3d3624e6b1c3e643197c2d
Reviewed-by: Christian Kandeler <christian.kandeler@theqtcompany.com>
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@theqtcompany.com>