Add a Version property to the Debug interface

Submitted by Will Thompson `@wjt`

Assigned to Telepathy bugs list

It'd be useful for the Empathy debug window to be able to tell you what version of Gabble, etc. you're running.

Maybe it should be something like:

( s: component_version, a( s: supporting_library_name, s: supporting_library_version ) )

so that Gabble could have:

( '0.9.1, [ ( 'wocky': '...'), ( 'tp-glib': '...') ] )

and similar for Flutterby etc.