Fix suggested file format on asymmetrical formats

In most cases, PD can save an image in its original format.

For some rare formats (e.g. various RAW incarnations), however, export
requires a new format.  Sloppy coding meant such images were being
recommended to BMP format, lol.
User-facing messages: display even when main form is disabled
Exporting images: display hourglass cursor as necessary

Previously, this was only happening on certain image formats.  Since
almost any format can take awhile to save if the image is large enough,
I should cover all formats.