The idea for this base class is to provide access to all elements which
make up a managed "Client" being it X11 or Wayland. They share a lot,
like they have a caption, they can be minimized, etc. etc.

Of course it would have also been possible to derive a new class from
Client, but that looks like the more difficult task as Client is very
X11 specific.

So far only a very small interface is extracted with pure-virtual
methods. This is going to change by moving the functionality up into
the AbstractClient.

The interface extracted so far is inspired by the usage of FocusChain
and users of FocusChain.