Support for Forth Syntax Highlighting

Thanks for creating this very wonderful effective control for Qt. I am an avid user of the QOwnNotes and a Forth enthusiast. While adding Forth code, I found that there is no highlighting support. I wish to request your kind help regarding the same. In line with this, I have taken a fork and placed in my changes. Initially, the `gforth` words are added as the keywords. There is also addition of special filter functions to help with comment highlighting. This would be the first time I am trying to contribute to a project. Please guide me and help me correct my mistakes.