A number of new objects are added as part of this commit to the
GtkSourceView ABI.

 - GtkSourceSnippet provides an object representing a snippet to be inserted
   into a textview. Snippets are associated with a textview rather than a
   buffer because of incremental state that is necessary to interact with
   widgetry and event controllers. Additionally, it doesn't make sense to have
   this attached to the buffer when the view area could be different.

 - GtkSourceSnippetChunk is a single chunk of a snippet. A snippet contains
   zero or more chunks. A chunk can have a spec (which can be evaluated
   using variables) or text set (such as after typing in the editor). Also, a
   chunk can have a "focus-position" which allows the user to tab through the
   chunks of the snippet.

 - GtkSourceSnippetContext provides state which can be expanded as part of
   the snippet. This is useful to expand variables set by the application or
  ...