Merge: gtksourceview: Add the missing `kwnull` context

Merge: gtksourceview: Add the missing `kwnull` context

Renderings for `test_syntax.nit`:
* [Classic](https://github.com/nitlang/nit/files/1148477/gtksourceview-classic.pdf)
* [Kate](https://github.com/nitlang/nit/files/1148476/gtksourceview-kate.pdf)
* [Solarized Clear](https://github.com/nitlang/nit/files/1148474/gtksourceview-solarized-clear.pdf)
* [Tango](https://github.com/nitlang/nit/files/1148475/gtksourceview-tango.pdf)

For comparison, see #2515.

![WHOOPS!!](https://user-images.githubusercontent.com/6044484/28213673-23e5b296-6875-11e7-8a2b-494652afe064.png "WHOOPS!!")

Signed-off-by: Jean-Christophe Beaupré <jcbrinfo@users.noreply.github.com>

Pull-Request: #2525