* Clear the trace and send the loadingFinished() signal when loading
  fails or is canceled. loadingFinished() re-enables the UI, which is
  in fact important.

* Check more consistently for whether the operation was canceled and
  add a separate signal for that.

* When saving fails or is canceled, remove the half-written file.

* Don't try to guess the number of events for progress reporting when
  saving. Use the event timestamps and start/end time instead.

* Properly initialize the progress range in all cases.

* Drop the bool return value from the load methods. Nobody uses that.

* Send loadFinished() only after loading a file, not every time we
  reach the Done state.

Change-Id: I507f11c667e20534ea335e84798de11aa06fb6f2
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>
Reviewed-by: Ulf Hermann <ulf.hermann@qt.io>