Fix progress bar not disappearing after image quantization
Fix status bar messages overlapping coordinate display

...when coordinates are very long (e.g. 5+ digits)
Image -> Tile now works with Layers

This completes the bulk of the work on getting the Image menu fully
functional with layers.  The only item left is Content-Aware Resize,
which I am debating moving to the Layers menu.  (I haven't decided the
best way to approach that one yet, so I am postponing it for the time
being...)