This adds a dropdown menu with completions from the completion index
when the user creates a new object, message or comment, and starts
typing. Note that this index is initially populated with objects from
the search index of the help browser. As the user types object names,
arguments, messages and comments, they will be added to the completion
index as well.

Entries from the menu can be chosen with the cursor up and down
keys. The enter key can then be used to select an entry and insert the
corresponding completion. Alternatively, clicking with the mouse also
selects an entry.

NOTES / TODO:

This is a very first implementation of autocompletion for purr-data, and
as such it still has a few minor quirks and shortcomings:

- Help index completions: Completions from the help browser's index will
  only be available once that index has been built. By default, this
  happens when the help browser is first launched. However, you can
  change this so that the help index i...