Implement Entity Markers

[Ben1138]

Implement entity markers, as used in the campaign. The respective Lua API calls are MapAddEntityMarker and MapRemoveEntityMarker.

Note that Entity Markers:

• Can be of any color
• Are also displayed as pulsating circles on the map
• Are displayed on the HUD corner when not in the view frustum (maybe with a padding?)
• There can be multiple active at once