Changed Configurable class

It makes more sense to assign real attributes once
instead of searching for proper value every time attribute is requested
If given attribute has a setter then it will be used
Don't use unmarshallable objects in stack's info()
s/(all|windows)/clients/g in the tests
send a synthetic ConfigureNotify event when position change

see 4.2.3. http://tronche.com/gui/x/icccm/sec-4.html