Rewrite window menu system

[lenemter]

Introduces WindowMenuManager that controls window menu items and what they do. This gives us a lot more control over window menus in general since now wm has proper callbacks support for window menu items. I plan to use it for #649. Plus this introduces ability to write custom plugins with this functionality (e.g. for #1493 because likely it won't be merged)

Allows us to deprecate Gala.ActionType in the future and cleanup DBus interface with the window manager implementation of it, but that should be done in OS 9.

[leolost2605]

@lenemter sorry that I never got around to do a review here, I always kinda skipped over it because I didn't have much time and it is a big diff. But I've got a week or two with plenty of time now so if you're still interested in this feel free to update it with the new changes from main and I'll be happy to do a review :)
Also I don't know if you've considered it and found it unfitting for this use case but I just want to quickly point to DBusMenuModel which we might be able to use here?

[lenemter]

@leolost2605 I didn't know about DBusMenuModel but I don't think it fits this use case. Window menu items can change depending on the window and I don't know if DBusMenuModel supports hiding/showing items changing descriptions on the fly, etc. Also it seems to be tied to actions which we don't use in gala in general