getSnippet
Queries the known snippets for the first matching @group, @language_id, and/or @trigger.
If @group or @language_id are null, they will be ignored.
Return
a #GtkSourceSnippet or null if no matching snippet was found.
Parameters
group
a group name or null
languageId
a #GtkSourceLanguage:id or null
trigger
the trigger for the snippet