Port to GTK4

[leolost2605]

This should work now.

Todo:

• window drag doesn't work on X11 (i'd like to treat this as not blocking because I'm not quite sure what's going on here and kinda consider X11 only legacy anyways but idk)
• The transparency doesn't fully work (probably a CSS issue idk)

Misc:

• We get a warning about gtk_is_ancestor receiving null everytime an indicator is opened but idk why and wouldn't consider it blocking i guess?

[danirabbit]

I think probably the best way forward for development is I can create a new packaging branch based on this branch and then build that in daily so we can start building indicators against it etc

But I'm not sure what trying to upgrade this mid-cycle looks like, so I'd like to leave it unmerged for now until we have a plan around how to handle the packaging/upgrade.

[danirabbit]

Don't forget to update deps on README and .github :)

[danirabbit]

There's a packaging branch now: https://github.com/elementary/wingpanel/tree/deb-packaging-gtk4

So we should have access to the library on OS 8 daily no problem to work with porting the other indicators when this builds: https://code.launchpad.net/~elementary-os/+recipe/wingpanel-8-daily

You might want to temporarily change the meson project name here to io.elementary.wingpanel-8 or something so that you can install them both at the same time for testing? Otherwise the io.elementary.wingpanel-8 package will refuse to install because of the file conflicts. But this package name will eventually be reverted anyways so that's optional I guess 🤷‍♀️