Introduce X11-specific and Wayland-specific sets of input event filters

On Wayland, features such as window actions and move/resize are implemented using generic input event filters. They work for all kinds of clients, e.g. XdgToplevelClient, InternalClient, etc.

The problem is that X11Client/XwaylandClient implements window management features as well using X11-specific stuff, which may conflict with the event filters.

In order to resolve that conflict, we need to introduce X11-specific and Wayland-specific sets of event filters and use only one of them based on whether we're running as a wayland compositor or as an x11 window manager.