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-theme-editor/src/main/java/com/formdev/flatlaf/themeeditor/FlatLafThemeEditor.java

@@ -46,6 +46,6 @@ public class FlatLafThemeEditor
 			System.setProperty( "apple.awt.application.appearance", "system" );
 		}
 
-		FlatThemeFileEditor.main( args );
+		FlatThemeFileEditor.launch( args );
 	}
 }

flatlaf-theme-editor/src/main/java/com/formdev/flatlaf/themeeditor/FlatThemeEditorOverlay.java

@@ -52,6 +52,7 @@ class FlatThemeEditorOverlay
 	private Font font;
 	private Font baseFont;
 
+	@SuppressWarnings( "FormatString" ) // Error Prone
 	@Override
 	public void paint( Graphics g, JComponent c ) {
 		// paint the syntax text area

flatlaf-theme-editor/src/main/java/com/formdev/flatlaf/themeeditor/FlatThemeFileEditor.java

@@ -102,7 +102,7 @@ class FlatThemeFileEditor
 	private final FlatThemePropertiesBaseManager propertiesBaseManager = new FlatThemePropertiesBaseManager();
 	private final JButton newButton;
 
-	static void main( String[] args ) {
+	static void launch( String[] args ) {
 		File dir = (args.length > 0)
 			? new File( args[0] )
 			: null;
@@ -1306,7 +1306,7 @@ class FlatThemeFileEditor
 				super.addElement( obj );
 			} else {
 				int index = binarySearch( this, obj, comparator );
-				insertElementAt( obj, (index < 0) ? ((-index)-1) : index );
+				insertElementAt( obj, (index < 0) ? (-index - 1) : index );
 			}
 		}