Implement LView logging deleted entities

[kfriedrichs]

All _getXXUpdates() functions (e.g., _getObjUpdates()) have no way of
recognizing deleted objects/targets/grippers: they check the received update
and compare to the internally saved last state, then compile a reduced
update out of any entities that differ between update and saved state.

• Detecting an entity was removed is easy, the _getXXUpdates() functions
  would simply have to check whether some of the entities in the saved state
  are missing in an update.

But the log consists of EXISTING entities with their updated properties.
How to log a no-longer-existing entity?

• One option would be to introduce a special type of log entry for this.
• Or every entry could have the 'event' key (the 'registeredLocalEvents'
  already have that) stating the type, like 'update' or 'deletion'.
  For example:

[
    2183, <- timestamp
    {
     "event": "update", <- type of update
     "grippers": {
       "fVHMwdLePCE2Vlq5AAAB": {
         "x": 9.5,
         "y": 10
       }
     }
    }
],
[
   2701,
   {
     "event": "deletion",
     "grippers": {
       "fVHMwdLePCE2Vlq5AAAB": { <- could also be reduced to id of entity
         "x": 9.5,
         "y": 10
       }
     }
   }
]