Rework word completion: Do not reparse whole document unnecessarily

[jeremypw]

This should only be merged after the code that implements dynamic updating of the word list (by detecting insertions and deletions rather than key presses) else the word list may get out of date more often (e.g. if doc changed externally).

The document is fully parsed once on loading, editing the document does not usually require a full reparse (unless forced).

• Add a flag to the prefix tree to indicate whether the associated document has undergone its initial, complete parse.
• Move timeout into on_new_sourceview and only call for unparsed documents