Work on improving pickable entities - Lights Optionally pickable

Some foundation work to improving picking of entities.

* introduced virtual method for individual entities to do detailed ray picking against their contents
* make "lights" choose to be pickable based on an EntityTree setting and Entities scripting interface that says whether or not they should be pickable
* added support to editModels and newEditEntities to toggle light pickability based on a menu item - lights NOT-pickable by default when those tools are running.

Additional things in this PR:
* cleanup of redundant _displayViewFrustum... this was recently added and wasn't actually adding any value, just complicating the code. moved back to using _viewFrustum
* added a hotkey to "reset" the offset view frustum... TODO: clean this up more, maybe migrate it to a developer script.
* remove dead code in MyAvatar for "mouseRay" and "mouseClicked" - these were not being used and really didn't belong in the avatar class anyway.
here's where the Tree/Element's findRayIntersection() code allows the entity type itself to do additional detailed picking. Eventually we will use this to allow models or other entities to do detailed ray picking as well.