Overhaul X11 window types

X11 windows are represented by two classes - X11Window and Unmanaged. One represents managed windows and the other represents override-redirect windows.

On one hand, the split makes sense, but on the other hand, such fragmentation is undesired. For example, when associating X11 windows with wl_surface; or creating platform specific items.

As an undesired sideeffect, the Window class is used to share code between X11Window and Unmanaged.

In principle, override redirect windows can be represented by X11Window. The main difference between OR and managed windows from kwin's perspective is that some operations are forbidden for OR windows.

I think that it's worth overhauling X11 window types:

• merge X11Window and Unmanaged
• move X11-specific code from Window to X11Window
• unify X11 apis in Workspace, e.g. Workspace::findClient() and Workspace::findUnmanaged(), Workspace::clientList() and Workspace::unmanagedList(), and so on