We need a shared window id in some way to e.g. properly support TabBox.
Though I do not like that the return value is xcb_window_t. This should
be either changed or a new "generic" windowId needs to be introduced.
If the ShellClient got created for a Qt internal window, we try to
find the QWindow and if we get one, we use the geometry directly as
it got set by KWin in the first place.
Also a windowId() is added to ShellClient which can be used by the
effect system to find an EffectWindow. If it's an internal QWindow
we just use that window id. For other clients we still need some
smart solution.
The ShellClient is a Toplevel subclass for a
KWayland::Server::ShellSurfaceInterface. It gets created when a new
ShellSurfaceInterface is created and destoryed when it gets unmapped.
So far the usage is still rather limited. The ShellClient is opened
at position (0/0). While it's possible to pass pointer events to it,
it's not yet possible to activate it, so no keyboard focus.
