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.
this is the new optional method that entities can implement if they are able to do more detailed ray intersection tests beyond just their bounding box which is handled automatically by the EntityTree/EntityTreeElement.