mate-screenshot: Restore aysnchronous saving

[cwendling]

Follow-up to #368 to restore asynchronous screenshot saving, but doing it more reasonably this time (before #368 it was using a exec()-less fork(), which is unsafe when using GTK libraries, leading to bugs).

[L-U-T-i]

I am getting the following build error with this patch:

/usr/bin/ld: /tmp/cc0yBufC.ltrans0.ltrans.o: in function `new_tmp_dir_async_ready':
/builddir/build/BUILD/mate-utils-c74d86d0f25435b1d90f70015d210448126bb6ef/mate-screenshot/src/screenshot-save.c:182: undefined reference to `g_file_new_tmp_dir_finish'
/usr/bin/ld: /tmp/cc0yBufC.ltrans0.ltrans.o: in function `screenshot_save_start':
/builddir/build/BUILD/mate-utils-c74d86d0f25435b1d90f70015d210448126bb6ef/mate-screenshot/src/screenshot-save.c:236: undefined reference to `g_file_new_tmp_dir_async'
collect2: error: ld returned 1 exit status
make[4]: *** [Makefile:566: mate-screenshot] Error 1

As much as I've been able to find, g_file_new_tmp_dir_finish and g_file_new_tmp_dir_async shall be available since glib2 2.74, while RHEL 9.6 is shipping with glib2 2.68.4 at the moment.

[cwendling]

@L-U-T-i ah, bummer. I just pushed a poor man's compatibility wrapper that seem to work fine. Tell me if that's still not enough.

@mate-desktop/core-team if the async directory stuff is getting too much out of hand, I can also drop it. Although nice, it used to be synchronous anyway.

[raveit65]

Mate-screenshot works with this PR in f43, but i do not judge which fix is better.
As lazy admin i prefer not to update a working fedora package again ;-)