Re-introduce missing Limechat copyright notice

The license under which Limechat's source code appears* to have been forked to form the basis of Textual stated: "[r]edistributions of source code must retain the above copyright notice". This makes your license compliant with that.

* https://github.com/psychs/limechat/commit/b23f9bb406071b3972461bbf1063906f179c367c