Bug fixes

- Removed NSWindow.WorksWhenModal.Set (this is not implemented in
NSWindow, only NSPanel).
- NSWindow Example: Added keystroke to turn off "Ignore Mouse Events".
Otherwise, turning it on meant that you had to close the window or quit
the app to continue.
- NSWindow Example: Locked some buttons on right instead of left to
better illustrate SmoothResize.
- PopupMenuExtension.Icon: Tests for a nil value and handles it.
- NSWorkspace.RunningApplications: Checked to make sure it responds to
selector, otherwise returns empty array.