Add a GspellTextTag class with a 'spell-checker' property
@swilmet
Submitted by Sébastien Wilmet Link to original bug (#760725)
Description
To support different languages inside the same buffer, a subclass of GtkTextTag with a 'spell-checker' property can be added to gspell.
(with the prefix 'spell-' to have a kind of namespace for the spell checking, since another 'checker' property could be added to GtkTextTag for another purpose).
Some utility functions will need to be written to retrieve the GspellChecker for a particular word in the buffer.
Why not the 'spell-language' property to store a GspellLanguage? Because a GspellChecker is needed, since it contains the session dictionary, and we want the same session dictionary per language (in the same buffer at least).
See GtkSourceTag for an example on how to subclass GtkTextTag. For gspell, a gspell_text_buffer_create_spell_tag() will also be needed, for convenience and consistency with how GtkTextTag are usually created.