Add a mechanism to test GTypes against QGlib::Types.

GTypes are unsigned longs so add an operator== to compare
against the GType hiding inside of QGlib::Types. I used it during some
debugging, though I'm not sure if its a good idea to leave it in.
Properly convert the void pointer being passed to the getter/setter

It was minor fixups from the original pointer conversions. I shouldn't have
removed the casts.
Use gst_caps_new_empty_simple instead of ...new_simple with NULL parameters.

Trying to use the more generic function with NULLs failed. So I also
added an extra test to check the case of a Caps::create call.

Lastly I simplified the media type string to not include the ,format=rgb
as it didn't work.