[allegro5] make the game Allegro 5 compatible

[ufoot]

A long running effort to backport what is required to make the game support Allegro 5. The strategy I'm generally following here is backport bits of Allegro 4 back into the game so that it has "just what it needs". Rewriting it totally in Allegro 5 spirit would be too complex, that would be a real rewrite, really. I tried. So far the most tricky parts are:

• the GUI system (this is more or less done, most of it)
• font support (a sub-problem of the above)
• all-in-one .dat files do not exist any more, so need to transition to folder, file system based resources
• no palette support (this is really a thing, since program relies heavily on this feature)

So far these are the "only" problems I identified.