app, plug-ins: remove file-uri completely

and enable the new file-remote code in app unconditionally.
app: separate the mount logic from uploading/downloading

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.
app: file_procedure_find(): check magics only for native files
plug-ins: start the progress before opening files for loading/saving

so the user gets feedback as early as possible.