editor: add GtkSourceCompletionWords.

This allows you to auto-complete words that are currently in the
document.

We still need a toggle switch to enable if we want this feature. We
probably want it to be able to be turned on globally, and disabled
on a per-language basis.
editor: wire a preference switch for word completion
vim: remove old TODO item
editor: wire enable-word-completion to editor setting.

This allows to enable/disable the word completion provider in the
editor settings.
action: fix reference counts on variant objects.