api: provide isTilePowered() to check whether a given location has power

instead of requiring front-end to test for PWRBIT

git-svn-id: http://micropolis.googlecode.com/svn/trunk@888 d9718cc8-9f43-0410-858b-315f434eb58c
cleanup: use constants for RivEdge table (in MapGenerator)

replace hard-coded numbers with constants and constants plus offsets

git-svn-id: http://micropolis.googlecode.com/svn/trunk@889 d9718cc8-9f43-0410-858b-315f434eb58c