MenuButtonSkin fails to initialize in a background thread, throwing an IllegalStateException calling Toolkit.getToolkit().getSystemMenu().isSupported() in MenuBarSkin:822

```
    @Test
    public void menuBar() {
        MenuBar c = new MenuBar();
        c.setSkin(new MenuBarSkin(c));
    }
```

## Root Cause

MenuBarSkin.rebuildUI() relies on Toolkit.getToolkit().getSystemMenu().isSupported() which can only be invoked in the FX application thread.

## Possible Solution

Postpone rebuildUI() until after the menu gets added to a Window (similarly to Chart fix https://bugs.openjdk.org/browse/JDK-8349091 ).