Add v1.2 support and catch up with gtk-rs master

Remaining issues

• girs feature handling is broken (docs are looking bad, ./fix-gir-mistakes.sh fixes this)
• gir does not generate a needed use statement (./fix-gir-mistakes.sh fixes this)

Diff

$ git diff master :^Handy-1.gir :^libhandy/src/auto/ :^libhandy-sys/{src,tests}