Work on improving pickable entities - Lights Optionally pickable, Spheres pick properly, other bug fixes

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.
* SphereEntityItem now correctly implements ray picking against it's sphere not it's bounding box.
* fix a bug in ray picking where we weren't always honoring the "closest distance" entity.

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.
use our SphereShape to do a better ray intersection test instead of our bounding box.