Fix moving type of moving platforms was not saved.

On saving, all moving platforms were saved as "line"-type
moving platforms due to the move_type attribute missing.
Fix mispelled CEGUI header names
Make m_callbacks protected so it can’t be accessed from the outside.

Introduce proper methods for this instead. External contexts should
never be allowed to mess with this variable (may result in segfaults).
Delete event handlers when reinitializing mruby interpreter.

This prevents segfaults when changing the script code.
Add doxygen for documentation.