UI: handle various entity click events

star : reset cam
jg / asteroid : focus on entity

Adds position_trackers to jg and asteroid js entities so as to
be able to used canvas#follow_entity
UI: add Canvas#descendants method to get children of all scenes

Use to fix regression caused by skybox being split out into
skyscene where unhover events (eg skybox hover events)
weren't being detected
UI: remove reset cam link (click star or system in list to do so)

Also a couple small changes to hide the entity container when the
cam is reset and to vertically align the toggle axis checkbox
UI: add system hover / clicked audio effects
UI: add new audio effects (generic command & confirm, docking)