Use pimpl in QtSlicer
Factorize code in QtSlicer
Rename QtSlicer into QtImageViewer
Cleanup QtSliceControlsWidget

Move the slider/spinbox setup into QtSliceControlsWidget.
Merge branch 'code-cleanup'

* code-cleanup:
  Cleanup QtSliceControlsWidget
  Rename QtSlicer into QtImageViewer
  Factorize code in QtSlicer
  Use pimpl in QtSlicer