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-demo/src/main/java/com/formdev/flatlaf/demo/DemoFrame.java

@@ -440,9 +440,9 @@ class DemoFrame
 
 		Class<? extends LookAndFeel> lafClass = UIManager.getLookAndFeel().getClass();
 		try {
-			FlatLaf.setup( lafClass.newInstance() );
+			FlatLaf.setup( lafClass.getDeclaredConstructor().newInstance() );
 			FlatLaf.updateUI();
-		} catch( InstantiationException | IllegalAccessException ex ) {
+		} catch( Exception ex ) {
 			LoggingFacade.INSTANCE.logSevere( null, ex );
 		}
 	}