Spelling fix
Added cache for highlighted minimap images.

Used for displaying the reach of a selected unit in the minimap.
Support for drawing highlighted regions.

Basic support for showing the reach of a unit or similar reach map
related issues in the minimap.
Lifted the reach map data structure from game_display to display.

The reach map is useful in the editor as well.
For now the map is used to show the highlighted reach in the minimap if
a unit is selected.
Put title screen hotkeys in there own scope.