Merge po files, set up for intltool and l10n.gnome.org

This removes the custom .pot file setup we had before, making it easy to
translate Shotwell using the standard intltool workflow.

This also merges the two separate core/extras gettext domains into one,
to keep things simple.  I hope I've merged all the po files correctly,
but I wouldn't bet money I did it perfectly.

We also set up the help to be able to be translated on l10n.gnome.org.

Closes: bgo #718783