added game panel classes to show items only before or after BD hatching

added game panel classes to show items only before or after BD hatching

Example: When using game panel definitions like

game.panel.levelset_name.class: bd_pre_hatching

or

game.panel.score.class:         bd_post_hatching

the corresponding game panel items will only be displayed before or
after the player was hatched in levels using the native BD game engine.
(For levels not using the BD engine, items using "bd_pre_hatching" will
never be displayed, while items using "bd_post_hatching" will always be
displayed.)