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