- always use Java toolchains
- default is Java 11 (but source/targetCompatibility is still 1.8)
- use system property `toolchain` (e.g. `-Dtoolchain=25`) to compile with other Java versions
preparation for Gradle 9.x, which requires Java 17+ to run
- 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)