Split out a dedicated InternalClient class

Summary:
Most of the functionality which is special to internal clients is moved
from ShellClient to InternalClient. As KWin's qpa is still bound to the
Wayland protocol InternalClient inherits from ShellClient. Due to that
some aspects in ShellClient are "weird". ShellClient still detects
whether it's an internal client and uses the variable m_internal to
capture the state. This is required as we cannot use the isInternal
method. Most of m_internal usage is in init which is called from
constructor of ShellClient. Thus it's not possible to call into virtual
methods of InternalClient.

Also some of the code is duplicated and some methods are temporarily
marked as virtual.

The next step will be to remove ShmBuffer for internal windows which
should decouple the two implementations further with the long term goal
of having InternalClient inherit AbstractClient directly.

Test Plan:
Run nested KWin, triggered outline (OpenGL case) and debug console (shm case).
InternalWindow unit test still passes.

Reviewers: #kwin

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D18569