Remove `graphics` (almost)

This merge request (almost) removes the graphics/ directory. The file pixel.h still has be to kept around a bit longer as it is used by dev/ppm/ppm.cc.

A decent number of commands were removed from ui/commands.cc and some of the user interface bits have been removed as well, for example ui/uginterface.{cc,h} and ui/cmdint.{cc,h}.