This is a replacement for GtkSourceCompletion to handle a number of designs
that we can't land into GtkSourceView for various API/ABI reasons but need
in Builder for completion performance, memory footprint, and visual
styling.

The important details in terms of design change are:

 * Use GListModel heavily to avoid GObject creation, GList creation,
   GQueue creation, and O(n²) treemodel lookups so that we can handle
   larger list sizes in (tens of thousands with much reduced memory
   footprint).
 * Allow providers to refilter existing results conveniently.
 * Break things into 4 columns (icon, lhs, word, rhs).
 * Use GtkListBoxRow instead of GtkTreeView for widget styling.
 * Only create widgets and proposals for the visible range of items
   through use of custom GtkBox-based GtkScrollable, listbox row, and
   dynamic access to joined-GListModel for all results.
 * Dynamic priorities for completion results based on context.
 * Adjust window size based on result ...