Added bindings support to GtkBuilder by introducing 3 new <property> attributes ...

Added bindings support to GtkBuilder by introducing 3 new <property> attributes "bind-source" to specify the source object of the binding "bind-property" to specify the source property and "bind-flags" to specify the binding flags (optional)

Binding an object sensitive property with a check button active property will look like this:

<object class="GtkButton" id="button">
  <property name="sensitive" bind-source="checkbutton" bind-property="active"/>
</object>

This is based on the original work done by Denis Washington for his GSoC project

This closes Bug 654417 "[GSoC] Add <binding> element to GtkBuilder syntax"