xgettext: Rewrite AppData scanner in ITS
* gettext-tools/its/appdata.loc: New file.
* gettext-tools/its/appdata.its: New file.
* gettext-tools/its/Makefile.am (dist_its_DATA): Add new files.
* gettext-tools/src/Makefile.am (xgettext_SOURCES): Remove x-appdata.c.
* gettext-tools/src/x-appdata.c: Remove.
* gettext-tools/src/x-appdata.h (EXTENSIONS_APPDATA): Set language
field to NULL.
(SCANNERS_APPDATA): Set func field to NULL.
