g_main_depth()

[Owen Taylor <otaylor redhat com>]

I went ahead and added g_main_depth() - I decided to make it 
global rather than per GMainContext because:

 - In the one use case we have, global seems equally good
 - Global gives us a little more opportunity for future 
   performance enhancements ... e.g., using __thread variables
   where available.
 - Fractionally simpler to document and think about.

Regards,
					Owen