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-extras/src/main/java/com/formdev/flatlaf/extras/FlatSVGIcon.java

@@ -279,7 +279,7 @@ public class FlatSVGIcon
 
 	private static synchronized URI loadFromStream( InputStream in ) throws IOException {
 		try( InputStream in2 = in ) {
-			return svgUniverse.loadSVG( in2, "/flatlaf-stream-" + (streamNumber++) );
+			return svgUniverse.loadSVG( in2, "/flatlaf-stream-" + streamNumber++ );
 		}
 	}
 
@@ -474,7 +474,7 @@ public class FlatSVGIcon
 		URI uri = this.uri;
 		if( uri == null ) {
 			URL url = getIconURL( name, dark );
-			if( url == null & dark )
+			if( url == null && dark )
 				url = getIconURL( name, false );
 
 			if( url == null ) {

flatlaf-extras/src/main/java/com/formdev/flatlaf/extras/FlatUIDefaultsInspector.java

@@ -770,6 +770,7 @@ public class FlatUIDefaultsInspector
 				return String.valueOf( value );
 		}
 
+		@SuppressWarnings( "FormatString" ) // Error Prone
 		private static String color2hex( Color color ) {
 			int rgb = color.getRGB();
 			boolean hasAlpha = color.getAlpha() != 255;