Add simple APPDATA_XML m4 macro

Validate AppData XML files by calling APPDATA_XML in configure.ac, and
then adding the .appdata.xml files to appdata_XML in a Makefile.am.
Finally, add @APPDATA_XML_RULES@ to the Makefile.am.

There is no support for translations here, and I am undecided about the best way to integrate those.