Display server and window interaction

[Fred Kiefer]

With the new split of the back end functionality a new class 
GSDisplayServer was introduced. As we may in principle have a lot of 
those, we must find the correct one for a given window, before we send 
it any window operation. This happens mostly in the NSWindow.m file and 
is an operation that need an extra mapping table and gives us the 
overhead of two function calls and one method call. The later could be 
optimized away if we directly supplied the window number to the 
GSServerForWindow() function.
But I want to question the existence of this function at all. Would it 
not be better to store the responsible server as an ivar in the window, 
and access it from there? This could be set in the _initBackendWindow 
method. To access and maybe change the server we could add to methods to 
NSWindow, but currently the only use for this could be the clean up in 
NSApplication.

For most operation that involve the Display Server we can be sure that 
the server ivar is already set otherwise the window number would also 
not be available. The remaining place where GSServerForWindow might be 
called without the ivar being set, involve the dragging, which is a not 
that common operation. So we could changed these to use the new method 
[NSWindow displayServer] that would return the default server, if none 
was setup.

Fred