xgettext: Fix --extract-all option in Glade2 files.

"context" and "comments" attributes from Glade2 files are stored in
the message list when --extract-all option is passed.
xgettext: GtkBuilder UI files extraction support.

It extracts every tag that has a translatable attribute.
Test GtkBuilder UI files extraction support.
xgettext: Document GtkBuilder UI files extraction support.