ScenarioV2 extension, sky map

[SJuliez]

This PR

• extends the YAML MM scenario description language substantially. It is now more flexible than the original scenario language w.r.t to game map definition; it also reads crew information (currently limited to single person crews) and various info for units (pre-deployed position, prone/hidden/shutdown/hulldown status, alt/elev and velocity, camo). It can now be used for setting up test games that continue working after the next few merges unlike savegames. The language example file is updated accordingly
• adds a "sky map" that is an atmospheric map without terrain as specified by the rules; (even with terrain, all terrain could be irrelevant for aeros). This is a visual representation of such a case: the sky map is just colored sky blue. Certainly subjective, to me this feels clearer than having a bunch of terrain (or an empty green map) under the aeros, where the scale of the terrain images is wrong anyway. This is currently only available from scenarios (but can certainly be added to the lobby GUI)
  
• fixes scenarion loading so that scenarios are now actually also read from the user dir

This is a fully functional little fight in the sky with pre-deployed units:

MMSVersion: 2
name: Air Battle
description: This is a fully functional little dogfight

map:
  type: sky
  width: 35
  height: 25

factions:
  - name: Epsilon Galaxy Flight
    camo: Clans/Coyote/Epsilon Galaxy.jpg

    units:
      - fullname: Cheetah F-11
        at: [2, 12]
        facing: 1
        altitude: 6
        velocity: 3
        crew:
          name: Marianne O'Brien
          gunnery: 4
          piloting: 4
          portrait: Female/Aerospace Pilot/ASF_F_2.png

      - fullname: Cheetah F-11
        at: [3, 17]
        facing: 1
        velocity: 2
        crew:
          name: Giulia DeMarco
          gunnery: 3
          piloting: 5
          portrait: Female/Aerospace Pilot/ASF_F_3.png

  - name: OpFor
    camo: Corporations/Star Corps.png
    units:
      - fullname: Cheetah F-11
        at: [ 32, 8 ]
        facing: 4
      - fullname: Cheetah F-11
        at: [ 34, 8 ]
        facing: 4

[HammerGS]

Tested and seems to work