cleanup: remove unused Avatar::_mode

The mode, if it is required, should probably eventually live in
AvatarDAta since it will be determined by whatever engine is running the
avatar (script or interface) and then broadcast to the rest of the
world.  When we need it we'll add it back in the right place.
move _thrust from Avatar to MyAvatar
remove unused getElapsedTime*() methods
remove unused MoveTarget stuff from MyAvatar
removing unused motion-state timers
temp disable of screen flash for avatar collisions
namechange CollisionFlags --> CollisionGroups
expose MyAvatar.collisionGroups to JS
expose COLLISION_GOUP_* constants to JS
add MyAvatar::_motionBehaviors (for obeyGravity)

Also reconcile Menu checkable set vs script-set collision groups.
add swissArmyJetpack.js
merge