0.6.8 Begin Level Mechanic

# Begin Level Mechanic
- Instructional text appears in the level before it begins.
- Player must use a Newton crit to destroy the text and begin the level.

# Bug Fixes
- Spawners now properly inform the Galaxy that they are destroyed.
- Galaxy checks for victory conditions when spawners are destroyed.
- Fixed a score bug where score was being incremented twice by
destroyed enemies.
- Changed some permissions on various fields.
- Exposed some fields as accessors via properties.