Gtk4Prep: Use Gtk.SourceCompletionWords

[jeremypw]

It is not clear why we implement our own word completion plugin when there is a provider already provided by Gtk. Our provider handler a wider range of characters but these are generally ones not allowed in Vala code names (and there are some flaws in our provider).

Pushing this for testing to see whether the Gtk provider is sufficient for Code, in which case a lot of code can be dropped and the porting made easier.

Converting our provider to GtkSource5 requires significant changes although not too hard.

If we are going to have our own provider it might be more useful to get proposals from the Vala symbols pane including info like function signatures and from relevant .vapi files.

Fixes #851
Fixes #1527

[danirabbit]

Yeah it looks like our word completion plugin is super duper old so maybe it just predates better completion in Gtk.SourceView.

I searched for issues related to word completion to check for regressions and none found! Let's go 🚀