Add initial support for Mutter 41

[decathorpe]

I'm getting a few warnings (pasted below), but it looks like this compiles and works as expected on Fedora 35 with Mutter 41.beta.

/WindowManager.vala config.vala
../compositor/WindowManager.vala:270.17-270.37: warning: Switch does not handle `UNMAXIMIZE' of enum `Meta.SizeChange'
                switch (which_change) {
                ^^^^^^^^^^^^^^^^^^^^^

compositor/io.elementary.greeter-compositor.p/WindowManager.c: In function ‘greeter_compositor_window_manager_real_plugin_info’:
compositor/io.elementary.greeter-compositor.p/WindowManager.c:1724:16: warning: function returns address of local variable [-Wreturn-local-addr]
 1724 |         return result;
      |                ^
compositor/io.elementary.greeter-compositor.p/WindowManager.c:1719:24: note: declared here
 1719 |         MetaPluginInfo _tmp0_;
      |                        ^

[decathorpe]

Thanks! 🚀