Add new element <template> and gtk_builder_expose_object() function to GtkBuilder

[Juan Pablo Ugarte <juanpablougarte gmail com>]

Hello guys, as part of my ongoing effort of bringing composite widgets
to Gtk+ I finished implementing a new GtkBuilder API
gtk_builder_expose_object() and added a new element to builder xml
format <template>

>From gtk_builder_expose_object() documentation

Adds object to a pool of objects external to the objects built by
builder. Objects exposed in this pool can be referred to by xml
fragments by specifying the "external-object" boolean attribute.

To make this function even more useful a new special entry point element
<template> is defined. It is similar to <object> with the only
difference it can only be defined as a toplevel element (that is it has
to be a child of <interface>) and its id has to reference an external
object exposed with this function. This way you can change properties
and even add children to an external object using builder and not just
reference it.

BTW there is a bug [1] in bugzilla to track these new features in
builder and I there is also a blog post [2] showing a bit more of what
can be accomplished using these new thing.

Any comments?

[1]https://bugzilla.gnome.org/show_bug.cgi?id=688205
[2]http://blogs.gnome.org/xjuan/2012/11/12/gtkbuilder-and-external-objects/