Add support for Mutter 43

[bobby285271]

This is a copy paste of:

• https://github.com/elementary/wingpanel/pull/453
• https://github.com/elementary/gala/pull/1451
• https://github.com/elementary/gala/pull/1457
• https://github.com/elementary/gala/pull/1327

This should work - i.e. This should compiles with mutter 43 and I can login using it. I cannot really test all cases.

Feel free to push something here or open another PR for this.

Hopefully closes #617

[tintou]

Can you update to use https://github.com/elementary/gala/commit/40bd02c621696c90287959260116d981039b9a57 instead of a09e356a1c26f8185f8788fadb2ec7b6aa49e32a from gala ? Other than that it looks good :)