Debugger: Always disable wait indicator when switching engines

The spinning wait indicator in the Locals view could go on infinitely
when the running debugger engine did not get a 'notifyUpdateFinished'
signal (e.g. if the engine quits between beginning and ending an update
cycle).

Change-Id: Ib3d7a88d3b89ed423b2d41e85a632ff01f91c155
Reviewed-by: Eike Ziller <eike.ziller@theqtcompany.com>