FlxNape: got SolarSystem demo back in action

### `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 =)