There has already been lots of code refactorings in gedit, to make the
code re-usable, clean it, don't use deprecated APIs, fix some problems
and improve the automatic/inline spell checker.

The *.ui files have been renamed, and the GResource path adapted for
gspell. There has been also a few minor changes to
gedit-spell-checker-language.c to be able to compile it. Apart from
that, it's an exact copy from gedit at commit
d74761bfa996bcf3a738e920e6830eadad63ac4f.