hotkey usability

resolve hotkey conflicts, fixes #2883
add/display existing in-game button hotkeys, fixes #2932
don't render empty remaining tick bars for iron curtain / chrono
construction yard HP reduced 2000 -> 1500

Now that MCV is buildable again, the conyard doesn't need to be the ultra-heavy armored bunker it has been. Now a nuke can almost kill it. 
increased speed of TowerMissile; inc. burst delay

Increased speed of TowerMissile. At present it is very slow and not terribly effective vs. air units because they can quickly leave range before getting hit. Speed stats are same for E3 and RocketBike.
Burstdelay increased. This is to avoid wasted missiles on the same target that might get destroyed.
Rangelimit reduced from 40 to 30 to compensate for increased missile speed. 
orca ROF decreased. 10 -> 15

Orca missiles fire a bit slower, so that the unit cannot unload all its rockets in 2 seconds. Will give defenses a bit more time to respond before they die or before the orcas run away.
mcv odds w/o base 80%

-Odds of getting MCV in a crate w/o a conyard is 80%, so that people aren't tempted to sell their conyard at beginning.
-Changed Hard AI's limits of procs, barracks & factories, since AI has to work within the conyard's build radius and crowding is no good, because it prevents them from building all tech buildings.
AI no longer builds FIX

To save space within conyard's build radius, AI will not build FIX, since it doesn't know how to use it, and it provides no pre-requisites. 
Added two new maps.
fixed error in slippery_slopes.oramap
Hotfix: make the C&C cheat menu not crash
more widgets for the C&C lobby

- random map
- crates on/off
- assign teams
Merge pull request #2961 from psydev/patch-2

construction yard HP reduced 2000 -> 1500
Merge pull request #2966 from Mailaender/cnc-cheats-crash

Fixes C&C cheats menu crash and adds more buttons to it's lobby
Merge pull request #2954 from Mailaender/empty-superweapon-indicators

Don't render empty remaining tick bars for iron curtain / chrono
added new map, skull_valley.
updated skull_valley map.
Merge pull request #2967 from psydev/changes

updated skull_valley map
use Mono.Nat for UPnP port forwarding

- might support more devices
- supports internal and external port mapping
- discover the device only once (at startup)
Better UPNP logging. Also fixes a compile error
Center C&C map chooser panel.