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)

flatlaf-swingx/src/main/java/com/formdev/flatlaf/swingx/ui/FlatTaskPaneUI.java

@@ -229,7 +229,7 @@ public class FlatTaskPaneUI
 
 			// compute chevron position
 			int cx = (int) (x + width / 2 - cw / 2);
-			int cy = (int) (y + (height / 2 - ch));
+			int cy = (int) (y + height / 2 - ch);
 			float offset = ch + UIScale.scale( 1f );
 
 			// set stroke with scaled width

flatlaf-swingx/src/main/module-info/module-info.java

@@ -14,7 +14,7 @@
  * limitations under the License.
  */
 
-/**
+/*
  * @author Karl Tauber
  */
 module com.formdev.flatlaf.swingx {