gui: make accelerator keystrokes adapt to operating system

keystrokes with string 'command' will be translated to
'meta' on Mac OS X and 'ctrl' for everything else

src/micropolisj/gui/MainWindow.java

@@ -429,9 +429,18 @@ public class MainWindow extends JFrame
 		}
 		if (strings.containsKey(prefix+".shortcut")) {
 			String shortcut = strings.getString(prefix+".shortcut");
-			menuItem.setAccelerator(
-				KeyStroke.getKeyStroke(shortcut)
-				);
+			KeyStroke keyStroke;
+			if (shortcut.startsWith("command")) {
+				KeyStroke key = KeyStroke.getKeyStroke(shortcut.substring(8));
+				keyStroke = KeyStroke.getKeyStroke(key.getKeyCode(),
+					key.getModifiers() |
+					Toolkit.getDefaultToolkit().getMenuShortcutKeyMask()
+					);
+			}
+			else {
+				keyStroke = KeyStroke.getKeyStroke(shortcut);
+			}
+			menuItem.setAccelerator(keyStroke);
 		}
 	}
 

strings/GuiStrings.properties

@@ -88,13 +88,13 @@ menu.game = Game
 menu.game.key = G
 menu.game.new = New City...
 menu.game.new.key = N
-menu.game.new.shortcut = ctrl N
+menu.game.new.shortcut = command N
 menu.game.load = Load City...
 menu.game.load.key = L
-menu.game.load.shortcut = ctrl O
+menu.game.load.shortcut = command O
 menu.game.save = Save City
 menu.game.save.key = S
-menu.game.save.shortcut = ctrl S
+menu.game.save.shortcut = command S
 menu.game.save_as = Save City as...
 menu.game.save_as.key = A
 menu.game.exit = Exit