Summary:
The idea behind this change is to have a dedicated binary for the
logout dialog. As a follow up ksmserver needs to be adjusted to invoke
the helper binary instead of interacting with KSMShutdownDlg directly.

This will bring quite some improvements to the logout architecture:
* ksmserver doesn't need to run a QEventLoop to wait for the dialog to finish
* it becomes easier to test changes for the logout dialog, one doesn't need
  to restart the session any more
* ksmserver becomes better guarded against problems in the UI layer (e.g.
  OpenGL errors)
* the binary can be changed to support Wayland
* ksmserver becomes better guarded against problems with updating Qt
  while the session is running

This change only introduces the creation of the binary. A new sub-dir
"logout-greeter" is added which creates a ksmserver-logout-greeter.
The greeter defines various command line arguments for the options which
are being passed to the KSMShutdownDlg...