data: Fix validation of GSettings schemas

The appdata translation added in
45b043865b3be7437fc9b0ae1ea08c8048668d84 uses the @INTLTOOL_XML_RULE@.
This causes GSettings schemas to also embed translations within
the XML file, which causes them to stop validating.

Move the appdata to a sub-directory to avoid the clashing XML
translation rules to appear in the same sub-directory.