Add an appdata.xml file.

* glom.appdata.xml.in: Add the file.
* configure.ac: Create glom.appdata.xml from the .in file.
  This lets us translate it.
* po/POTFILES.in: Mention the file so its contents can
  be translated.
* Makefile.am: Install the file next to the .desktop file.

This is needed by the new GNOME Software Center.