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)

.github/workflows/ci.yml

@@ -43,6 +43,10 @@ jobs:
           distribution: adopt  # Java 8 and 11 are pre-installed on ubuntu-latest
           cache: gradle
 
+      - name: Check with Error Prone
+        if: matrix.java == '11'
+        run: ./gradlew errorprone clean -Dtoolchain=${{ matrix.toolchain }}
+
       - name: Build with Gradle
         run: ./gradlew build -Dtoolchain=${{ matrix.toolchain }}