Handle the shortcut key like the meta key

KeyCombination.shortcut is a way to add platform-independent keyboard
shortcuts that will e.g. use "Ctrl" on Windows and "Cmd" on macOS
(see the Javadoc in javafx.scene.input.KeyCombination).

This commit just adds a check for getShortcut() and sets the
NSEventModifierFlagCommand just like is already done for getMeta().

Fixes 0x4a616e#24

src/main/java/de/jangassen/platform/mac/convert/MenuItemConverter.java

@@ -124,7 +124,8 @@ private static int toKeyEventModifierFlags(KeyCombination accelerator) {
     if (accelerator.getAlt() == KeyCombination.ModifierValue.DOWN) {
       modifiers |= NSEventModifierFlags.NSEventModifierFlagOption;
     }
-    if (accelerator.getMeta() == KeyCombination.ModifierValue.DOWN) {
+    if (accelerator.getMeta() == KeyCombination.ModifierValue.DOWN ||
+        accelerator.getShortcut() == KeyCombination.ModifierValue.DOWN) {
       modifiers |= NSEventModifierFlags.NSEventModifierFlagCommand;
     }
     if (accelerator.getControl() == KeyCombination.ModifierValue.DOWN) {