Expose class ids in XfwWindow and XfwApplication

The term class id seems quite appropriate, since it is of course used on X11 in WM_CLASS, but also on Wayland, at least in the xdg_toplevel::set_app_id doc, even though we're actually retrieving the app-id from the wlr-foreign-toplevel-management protocol.

Closes: #15 (closed)