[bug-gettext] GtkBuilder support

[Daiki Ueno]

So, the next step would be to add support for GtkBuilder files.

Before starting the review, I would like to ask a very basic question.

Are there any other differences between Glade 2 and GtkBuilder files,
except the following?

1. the msgctxt can be specified through context="ctxt" attribute
2. <col> and <attribute> elements may have translatable content

If no other, I think it would be better to simply add those features in
an upper compatible way, instead of rewriting x-glade.c parser to handle
Glade 1, Glade 2, and GtkBuilder file types separately (and strictly).

I mean:

For 1, we could just set a global variable allow_glib_context to true,
if a top-level "glade-interface" (which implies Glade 2) is detected,
and treat the context attribute accordingly.

For 2, just allow those new elements as well as <property> and
<atkproperty>.

I'd expect the minimal patch in this direction will be less than ~100
lines.

By the way, by grepping the GTK+ source, it seems <item> and <mark>
elements are also translatable.

Regards,
-- 
Daiki Ueno