[patch] fix order-dependent construction parameters in DBusGProxy

[Rob Taylor]

Robert McQueen wrote:
> Hello? Anyone? Good? Bad? Ugly?
> 
> Robert McQueen wrote:
>> Attached is a patch to allow constructing derived classes of DBusGProxy
>> if you don't specify the connection last. Otherwise it segfaulted
>> because setting the connection property tried to register with the
>> DBusGProxyManager, which relies on the other properties being set. Moved
>> this to the constructor and added some asserts for related issues. OK to
>> apply?

Looks good to me.

Rob Taylor