Bug 706221 - File/Save (As) dialog: add a link to the Export dialog...

...for Non-XCF files.

When the entered extension in save or export is on the other group,
add a link to the warning dialog which allows to jump directly to the
export or save dialog, with the same filename pre-entered.

(cherry picked from commit b56145626e1e83ed9e1ffea161fa66aadb9fea24)