Rename Gtrue and Gfalse to GTRUE and GFALSE

To comply with #[warn(non_uppercase_statics)]

This will/would break everything outside of rgtk using those constants but I think it would be easily enough to fix in any project and seems like the correct thing to do for the future (to comply with rust coding convertions)
It seems you have some git conflict here, this break this should break the build