Adding a build option for KWin Scripting

It is possible that adding this build option broke the Scripting
component. This is something that should not happen. Unfortunately
by just ifdefing everything scripting related with scripting enabled
we have build errors. These are caused by the fact that the scripting
code includes e.g. client.h through "./../client.h". At one offending
place I changed that to "client.h", but there is also a client.h in
the scripting directory.

The includes and naming of the scripting files clearly have to be fixed!