GtkTextView: Add fallback and letter-spacing support

Add support for the Pango attributes controlling font fallback
and letter spacing to GtkTextTag.

https://bugzilla.gnome.org/show_bug.cgi?id=740954
Allow inserting Pango markup in text buffers

This has been a very long-standing feature request.

https://bugzilla.gnome.org/show_bug.cgi?id=59390