Add a Progress Bar when Stitching Panoramas

[microbug]

When I click 'Stitch Panorama' in the Panorama dialog, there is often a wait of ~1 minute per panorama for the stitching to complete (working with large TIFF files). A progress bar would be helpful here so that the user knows the program is making progress. There is sometimes a spinning progress wheel in the centre of the panorama dialog when stitching panoramas, but it doesn't always appear — this may be a bug.