- replaced maven badges with shields.io because maven-badges.herokuapp.com did not show latest version 3.6.1 (instead shows 3.6)
- also the link target search.maven.org does not show 3.6.1
- now link to central.sonatype.com, which seems to be the successor of search.maven.org
https://central.sonatype.org/faq/what-happened-to-search-maven-org/
- 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)
