Add GUI option to import .lang files

## Prerequisites
- [x] I have searched open and closed issues for duplicates.

## Feature

As far as I can tell, Code uses `gtk-sourceview` for syntax highlighting. Therefore, it should be easy to add new language definitions by simply placing the `.lang` and a mimetype file in the appropriate folders. However, Code does not offer a way to it through it's GUI. Instead, users have to manually install the language definition, or file a Github issue to get their favourite languages supported.

My solution is simple. Include an option for the user to import `.lang` files. It could be placed in the _Syntax Highlighting_ menu. If there is already a mimetype file installed that references the new language definition, then that mimetype would be used. Otherwise the user would be prompted to create a new mimetype.

### Alternatives

* Have the user manually install new language definitions.

* Have the user file an issue in this repo to get official support for his language of choice.