GTK Upgrade

Wraps most of GTK to make it easier to upgrade (possibly to something else) Replaces uses of deprecated symbols with non-deprecated ones

No functional changes (intended)

Given the size of the diff, if it builds and runs on each platform (with no regressions compared to master) it's probably okay

Builds and runs on:

• Arch
• Manjaro
• Ubuntu
• Fedora
• MSYS2

MSYS1 has issues, using the gtk, gdk and fontconfig dlls from MSYS2 works though