Toplevel::window() is the actual X11 window. This makes it difficult
to use as the generic identifier for both X11 and Wayland. The Wayland
ShellClient already had a windowId() which is now added to Toplevel as
a virtual method. On X11 (Toplevel default) it returns the window().

The method window() now returns XCB_WINDOW_NONE for classes without
the Toplevel::m_client, such as ShellClient. Thus it allows to properly
check whether we are on Wayland or X11.

The code is adjusted to use windowId where a generic id is needed and
to properly check whether the window is valid before using it where
a window() is used.

This also fixes at least one additional unknown issue in
Workspace::setActiveClient

where the windowId of a Wayland client was passed to X11.