Create a completion_index for the autocompletion feature

- create the completion_index at the initialization of purr data
- add newly created objects to the completion_index
- update the completion_index by adding all objects from the index
  of the help browser whenever the index is created via make_index
- the completion_index is saved into a json file when purr-data
  quits, and also every 50 changes in order to mitigate data loss
  in case purr-data crashes