- CI runs Error Prone with Java 11
- use Gradle task `errorprone` to run it on development machine
- fixes are mostly cosmetic except:
- use Locale.ENGLISH for String.toLowerCase()
- use explicit character encoding when reading/writing files
- TabbedPane: wrong logic in mouse-wheel scrolling
- SplitPane: simplified property change listener (fixes hiding field `propertyChangeListener` of superclass)