Add macOS support

See merge request GNOME/eog!143

The general save callback and the specific callback for saving images on
exit both clear the window's save job. The second callback triggering
will then cause a critical warning. Instead of manually clearing the
job, use `g_clear_object` which avoids that warning.

Fixes #263.

`eog_window_get_image()` can return NULL even though the window returns
that it isn't empty. So it's sufficient to check that we actually got
an image from the window to avoid a NULL reference next.

Part of #263.

Otherwise the jobs' finished callback could be invoked after the store
is finalized.

Part of #263.

Emitting the "finished" signal happens delayed through the mainloop
while the job is executed async by the JobScheduler. In the time between
queuing the signal emission until it happens the job could be cancelled.
Do not emit the signal then as the caller is obviously no longer
interested in the job's result.

Part of #263.

Avoid critical warnings when saving images on exit

Closes #263

See merge request GNOME/eog!142