FlxNape: got SolarSystem demo back in action

[D-side]

FlxNape

Minor changes to get SolarSystem demo back.

• Number of states in demo extracted into a constant statesNumber for convenient changes in the future
• Prettier prevDemo function in Main and overall refactoring to use statesNumber
• Some changes in SolarSystem class to make it work with the new structure
  • inheritance from FlxNapeState changed to BaseState
  • no key check for G, R and arrow keys: done in BaseState anyway
  • halfWidth and halfHeight are now filled on state's create: otherwise stay at 0

Changes are minor, not feeling any platform-specific quirks in effect here. Tested out on Linux/C++ target and Flash, works fine.

My first pull request ever. Might have screwed up somewhere =)

[D-side]

For some reason, crashes on launch on Neko. Before and after the change. Investigating.

[Gama11]

Nice! I think this part of the demo was never entirely finished, and thus not used (it's missing graphics, but the phyiscs work well enough without that). Might be able to find something suitable on http://opengameart.org/ or similar...

About neko, don't worry about it. It's been discussed here. It's probably some crazy neko bug.

My first pull request ever. Might have screwed up somewhere =)

Nope, everything's perfect. :)

[gamedevsam]

Congrats D-side! If you keep it up we'll make you part of the team 👍