We need to mount every non-native GFile's volume, regardless of
whether the file procedure uses GIO directly or needs file-remote to
download/upload the file. Move the entire mount logic to a new
file-remote function and change gimp-gui.c and gui-vtable.c to only
return a GMountOperation. Try to mount every non-native file in
file-open.c and file-save.c and bail out if mounting fails. Simplify
the uploading and downloading code accordingly.