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.
There's never a time when we'd want the "display frustum" to be out of sync with the "view frustum" - this display frustum was added in an effort to fix some light issues with oculus. The original bug was caused by the view frustum being used by lights not being updated for each eye's vantage point. But the better fix is to actually update the _viewFrustum member since it also needs to be per eye accurate so that correct view culling will occur for other rendering aspects of the scene.