Add support for extracting strings from GMenu markup in GtkBuilder
files.

This means that we have to support translatable <attribute/> tags.

Unfortunately, <attribute> is also used by GtkTreeViewColumn and
GtkLabel for other purposes, and those other purposes use a value=''
(XML) attribute, so we must accept (and ignore) that in order not to
have errors.  Nothing will happen in those cases because they do not
also specify translatable='yes', so we ignore them.

https://bugzilla.gnome.org/show_bug.cgi?id=720552