Revert "add FixPt data type to replace some longs"

This reverts commit a2523a503816906a673fc6d119f9fd656c334794 because there
was too much of a performance hit.

Conflicts:
	src/Sim.cs
more tweaks after revert
pressing + or - changes game speed
pressing shift shows final positions of selected units
move scenario loading code into scnOpen method