Fix a lot of warnings in GTK4

Some of these warnings actually pointed at real errors, these are fixed too.

The only ones left are puzzle warnings (i left these alone), one printing warning (don't want to touch printing code) and two places with deprecated calls (still need to figure out what to use in their stead, but that's a fight for another day).

Most warnings are in W32 only code (which is why they went unfixed for so long).