Fixed a bug with system visibility...

....assigning multiple systems to a room, unassigning them, then
assigning them again would cause the inactive systems to become visible
when the room was moved/resized
Changed hotkeys to use Character.toString(char)...

...instead of char casting to determine the character represented by the
code point,
Added some documentation.
Update readme.developers.txt ...

...further explained the purpose of props with an example