nitpicker: Perform redraw asynchronously

This patch changes nitpicker's way of redrawing. Originally, redraw
operations were triggered immediately by the RPC functions invoked by
clients. In the presence of clients that invoked a large number of those
functions, the server could become overloaded with processing redraw
operations. The new version performs redraw operations out of band with
the RPC functions. Similar to the design of the DOpE GUI server, redraw
operations are processed periodically. The RPC functions merely modify
meta data and track the dirty areas that need to be updated.
Consequently, nitpicker's RPC functions become light-weight operations.

As a nice collateral effect of this patch, nitpicker's internal
structure could be simplified because the drawing backend is no longer
needed by the code that dispatches the RPC interface.
Scout: Add DroidSans-Bold10 font as tff
Minimalistic decorator for window system
decorator stress test
Simple desktop environment (WiP)