added Error Prone (https://errorprone.info/) and fixed reported errors and warnings

- 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)

gradle/libs.versions.toml

@@ -54,6 +54,12 @@ junit-api = { module = "org.junit.jupiter:junit-jupiter-api", version.ref = "jun
 junit-params = { module = "org.junit.jupiter:junit-jupiter-params", version.ref = "junit" }
 junit-engine = { module = "org.junit.jupiter:junit-jupiter-engine", version.ref = "junit" }
 
+# errorprone
+errorprone = "com.google.errorprone:error_prone_core:2.19.1"
+
 
 [bundles]
 junit = [ "junit-api", "junit-params" ]
+
+[plugins]
+errorprone = { id = "net.ltgt.errorprone", version = "3.1.0" }