qscintilla2 2.8.3 with designer plugin

Update QScintilla2 from 2.8.0 to 2.8.3

This is the last, I think, widely used Qt-dependent lib that builds/installs a Qt Designer plugin (others: [qwt](https://github.com/Homebrew/homebrew/blob/master/Library/Formula/qwt.rb), [qwtpolar](https://github.com/Homebrew/homebrew/blob/master/Library/Formula/qwtpolar.rb)). After this is pushed, can then focus on [making a common `plugins` directory for `qt`](https://github.com/Homebrew/homebrew/pull/26152#issuecomment-51350337) and reset the Designer plugin symlinks to there (instead of `qt`'s Cellar) for the above formulae, once completed/tested.