Mission availability refinements

[rikus]

I don't think it matters how soon this gets done, but I wanted it to be on the list.

(1) When you accept a mission, you shouldn't be able to immediately accept that same mission again (and again, etc.). Maybe it would disappear from the list when that becomes possible in the UI.

(2) Availability of missions could be influenced by more than just random numbers. For example:

• Some consistency in the list between landings (not just a new random set each time)
• Influenced by alliance, proximity to other planets, etc.
• Accepting missions could cause fewer to be available on the planet for a short while so there's not an endless supply
• Story missions could depend on completion of other story missions

[knowknowledge]

Regarding #1, What do you think of just closing that mission's Accept button when the mission is accepted:

-				UI.newButton( width-190, 20, 100, 20, "Accept",  string.format("PLAYER:AcceptMission(%q, availableMissions[%d])", missionType, i) )
+				UI.newButton( width-190, 20, 100, 20, "Accept",  string.format("PLAYER:AcceptMission(%q, availableMissions[%d]); UI.search('/Window/Tabs/Tab[3]/Frame[%d]/Button/'):close()", missionType, i, i-1) )

This isn't necessarily pretty enough though. We may want to add change the way that the Mission Frame looks or add an "Accepted" Label.