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
   filters to transform input text or other variables.

 - gtksourceview-snippets.c contains integration bits to be hooked into
   GtkSourceView. Keeping much of this outside of gtksourceview.c helps
   to keep things mostly self-contained but also ensures that we don't keep
   growing gtksourceview.c with complexity and size. Future additions to
   gtksourceview.c should be done this way when it makes sense (such as
   adding indenters).

 - GtkSourceSnippetBundle is used for multiple purposes because it is handy
   to do so without increasing the number of GObjects we need and because
   it flows naturally. It is used to parse new snippet files as well as
   merge multiple snippet files together. Furthermore, it serves as a
   GListModel with a filtered set of snippets when queried by the
   snippet manager. This type is private, however.

 - A completion provider is provided so that applications can use snippets
   provided by the snippet manager, however it still needs porting to the
   new completion engine.

 - Tabbing will expand the snippet based on the current word.

 - The classic style scheme has been adjusted to give us access to a focus
   position tag so that they are highlighted to the user. Style schemes
   bundled with GtkSourceView will want to implement this in a future commit.

 - Applications can insert snippets using gtk_source_view_push_snippet().

 - Using the mouse or touch input to move to another chunk will cause it to
   be focused (and selected). Moving between chunks manually will cause the
   snippet to be released.

 - A number of snippet variables are made available as seen in other snippet
   engines, notable textmate and Visual Studio Code.

 - You can toggle snippets on/off in test-widget using a checkbox. The
   snippets from data/snippets/ are available based on the current
   language.

 - Documentation for the file format and snippet text format is provided
   as part of the gtk-doc installation.

 - A snippets.rng file is provided to validate snippet files

 - Various style schemes have gotten "snippet-focus" styles that apply
   to the focus positions of the snippet.