Clean up of backend code

[Fred Kiefer]

Hi Adam,

before starting the MS Windows backend I would like to clean up the code
of your current backend a bit, to make it easier to extract the common
code from the xgps backend. Your last changes here where a big help in
that regard. I would like to go even further and remove the drawing
engine file. In the xgps backend the remaing methods here could be
merged into the XGContext.m file (The NSBeep method should go in to
SharedX/XGContextWindow.m and the frame link functions should be
deleted)
If the implementation of NSCopyBits is correct than this should be a
function in the frontend and for the xdps implementation that code
should be moved into DPSComposite. To implement NSCopyBitmapFromGState
we need another drawing interface in NSGraphicContext, propably
something similar to the interface we need for the new NSImage drawing
methods.

Of course the same changes should be made for xdps as well.

Cheers
Fred