xdg_shell: add missing <description> tags to the 'state' enum entries

The scanner parses CDATA in <entry> but lets it disappear otherwise. To have
descriptive text associated with the <entry>, we need a <description> tag -
and that must have a summary attribute. The current scanner doesn't handle
<description> however, so to get the summary printed in the output file we
still need the summary attribute in the entry tag too.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Reviewed-by: Derek Foreman <derekf@osg.samsung.com>
Reviewed-by: Jonas Ådahl <jadahl@gmail.com>