Relicense with GPLv2+

[jefferyto]

This plugin is currently licensed under GPLv3+. To be 100% compatible with gedit, I'd like to change the license to GPLv2+.

@Habetdin: would you agree to have your contribution relicensed under GPLv2+?

[Habetdin]

@jefferyto yes, sure.

I also will update the translation later.

[jefferyto]

Thanks @Habetdin!

I also will update the translation later.

The new strings are all from the settings schema; I don't think they are visible to the user (to be honest I'm not entirely sure they should be included in the .pot file) so I'm not sure there is a big need yet for them to be translated.