RC3: Colors for messages on worldmap

[HaJo]

It would be nice to color-code the messages that appear on top of worldmap,
e.g. red for danger/new alien activity,
green for important no-danger (mission solved, building/research complete),
white for routine messages (aircraft start/arrival), etc.

[HaJo]

Issue 1532620 on sf.net:
https://sourceforge.net/tracker/index.php?func=detail&aid=1532620&group_id=157793&atid=805245