app, libgimp, pdb: Allow .xcf.gz as extension in gimp_image_set_file()

Commit fc2925de restricted the extension to .xcf, which broke Gimp.file_load for .xcf.gz files.

Note: That commit was a reaction to a bug in the guillotine plug-in. I tested the plug-in with this fix, and the generated filenames are of the form "test.xcf-0-0.xcf" for "test.xcf.gz", as only the last extension is removed. This is probably good enough for now.

Resolves #10890 (closed)