Experimental GTK structs-as-references support. Not enabled by default.

Experimental GTK structs-as-references support. Not enabled by default.

This is still missing methods-with-ref-args. Adding them w/o
switching to using reference types is too dangerous, and not
really an improvement as long as we have to work with pointers.

The reason for not switching right now is GDC -- currently it does
not return small structs (in our case just a pointer) in registers,
making the GTK constructors more expensive than they need to be.
Maybe this feature can be enabled anyway, but for now let's wait.
Turning this on is backwards compatible (does not require any changes
in applications, as the ref structs implicitly convert to pointers);
turning if off *would* break apps.