This change does away with separate attributes foreground and background,
opting for an array of color "aspects", where the aspect can vary
over an enum which can either be FOREGROUND or BACKGROUND. This will allow
writing a single set of color-editing code which can work either on the
foreground or background color of a cell.
Now palettes are allocated and deallocated in fteapot as well as teapot;
both executables have some color rendering, with fteapot using the cell
foreground and background (there's just no way to set them at the moment).
teapot is so far only using color in the header.
Implements #39.
