Dynamically update word completions

[jeremypw]

Based on #1487

• Plugin: Handle 'insert-text' and 'delete-range' signals to update prefix-tree before buffer changes
• Engine: Only use split-set with custom delimiters to identify words (do not use textiter functions)
• PrefixNode: Split out into separate file
• PrefixNode: Add functionality to record multiple occurrences of word.
• PrefixNode: Add link to parent node
• PrefixNode: Move code to insert, find, remove and list completions here
• PrefixTree: Manage reaping of unused words, interface with engine.
• CompletionProvider: Off-load some functions to parser for consistency
• CompletionProvider: Completion overwrites remainder of word when inserting in middle
• Discontinue key press handling (no need to force tree update?)
• General: Update code style and headers. Add extra comments and debug output.
• General: Adjust minimum length of word and minimum length of prefix
• General: Introduce maximum length of word stored
• Show completions after deleting with backspace

For simplicity, insertions and deletions are all handled the same way rather than separate functions depending on where the insertion/deletion occurred. This results in some temporary "words" being added and removed in quick succession while typing but on current average hardware (i5 processor, SSD) and modest sized files (3000 lines) no performance issues were noticed. However, more efficiency could be achieved at the cost of greater complexity by queueing the changes (like Files) and ignoring those that cancel out.

To be decided:

• Best order for completions
• Best values for min prefix and min word
• Best value for interactive delay
• Whether forced update should still be availabe

[jeremypw]

Flatpak CI failing due to Gnome Gitlab outage.

[jeremypw]

I'm going to dog-food this for a while before asking for reviews.

[jeremypw]

@colinkiama If you have time to try and break this I would be grateful! The logic is quite involved so it is hard to be sure it copes with all circumstances.

[jeremypw]

At the moment this only works reliably in OS7.1. There are some weird bugs when running it on OS8 inside a Gnome Boxes VM. Not sure why - maybe a timing thing? Converting back to draft while investigating.