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.
The internal used window Id consists of two parts identifiying the
Client and one identifying the Surface. That is the first 16 bits
are set to the ClientConnection, the last 16 bits are taken from
the Surface id. As the Surface id is 32 bits, but we only use 16 bits
there is a chance of overlap. So this might need some improvement.
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.
