[Wayland] Add support for xdg-toplevel-tag-v1 protocol

[Sunderland93]

Implement support for xdg-toplevel-tag-v1 protocol, which is useful for window rules (e.g. in Kwin) or restore window position and size after restart application.

[ColinKinloch]

It's a bit of a nitpick but I'd go for something like:

xdg_toplevel_tag_manager_v1_set_toplevel_tag(
  wl->xdg_toplevel_tag_manager, wl->xdg_toplevel,
  WINDOW_TITLE " main window");

That way it can be easier to make distinct from the qt companion window if that gets tagged in the future.

Maybe renaming WINDOW_TITLE to DEFAULT_WINDOW_TITLE would be a good idea also.

[Sunderland93]

Yes, you're right. According to the protocol, the tag is an additional toplevel identifier, in addition to the app_id, and doesn't replace it. It would be more correct to simply specify "main window" as the tag, as it should be untranslatable. And the window title should be included in the description.