SDL interface reorg checked in

[Stan Shebs <shebs at shebs dot cnchost dot com>]

After repeated delays, the first stage of the improved SDL interface
is checked in.  Visually, it doesn't look much different, except for
a unit info area at the bottom.  Internally, it's quite different;
there is a new file sdlscreen.c, and new types "Screen", which takes
over most of what is in "Map", and "Panel", which is the panel type
that's been discussed previously.  Quite a bit of the interface has
been touched by renaming "map" to "screen", but this was mostly a
mechanical transformation.

The code is still in its early stages, doesn't have all the flexibility
that we've talked about, there are still no popups, and it hasn't yet
been tested on Macs, so it's probably broken there (sorry Hans).  But
the new structure should allow for building the full-scale interface.

Stan