Filling in the SDL interface

[Stan Shebs]

I just checked in a very basic implementation of buttons, plus a
use of them for construction.  The panel layout is similar to that
of AoE; if there are too many buttons to fit the space, there are
buttons created (labeled "Prev" and "Next") that you can click on
to see more possibilities.

Between that and the recently-added world map display, the SDL
interface is getting almost playable!

BTW, if you try it out and see pyrotechnics around the edges of
panels or buttons, don't worry about it, it's debugging code that
randomly recolors borders every time they're redrawn.  Very
useful to detect unnecessary redraws, which is the bane of performance
for this interface.

Stan