[autotest] Welcome to integration testing KWin

This is the beginning of a new testing era for KWin: finally we are
able to test against a running KWin. This works by making use of the
new virtual framebuffer backend for Wayland. It starts a specific
Application subclass which is mostly a fork of ApplicationWayland.

The individual tests are able to influence the socket name and the
size of the virtual screen. This is supposed to be done in
initTestCase. To know when KWin is fully started one can use the
workspaceCreated signal of KWin::Application. KWin is not started in
another process, but the kwin library is used, so the test has pretty
much full introspection to everything going on inside KWin. It can
access the Workspace, WaylandServer, fake input events through
InputRedirection and so on.

Once the test KWin is running it's possible to connect to it using
KWayland::Client library. This allows to introspect the Workspace
to see whether all worked as expected (e.g. correct stacking order,
active window and so on).

This first autotest is mostly meant to illustrate how to setup a
test and how one can use KWayland::Client to interact with the mock
KWin. For more tests it is suggested to move the connections to the
Wayland server in the init() and cleanup() methods.

The change also affects the qpa plugin: the specific check to only
run in binaries called kwin_wayland doesn't hold any more. This can
now be overwritten by an env variable.

Please note that this first test will probably fail in the CI system
as it might not have XWayland which is needed by KWin.