MRView capture tool: prevent frame index reset when no change in prefix
MRView capture tool: prevent frame index reset when no change in prefix

Previously, simply placing the cursor in the prefix entry in the Screen
Capture tool, then moving the focus away from it would cause the frame
index to reset to zero, since this would trigger the editingFinished()
signal. Now the slot will only be invoked if the text in the entry has
actually been modified...

MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3