Glade didn't show any LibreOffice controls in its tool palette
because the catalog didn't have any glade-widget-group element.
This adds a XSLT script to automatically generate such element.

A better version of the script would create separate groups for
each module/namespace, but currently I have no idea how to do
this with libxslt, as it doesn't provide fn::distinct-values().

Change-Id: Iaa3e4b0471b58c48838e000e6d78a73b0c6a4bd3
Reviewed-on: https://gerrit.libreoffice.org/2724Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>