Use popover controller like a custom dialog popup

This request contains two enhancements:

1. Added a None arrow direction type so that no arrow is drawn and so that the popup view is NOT anchored to some rect on the screen.  Instead, the popup view is always centered on the screen
2. Added special presentPopoverAsDialogAnimated: method so that the caller doesn't need to specify a rect to anchor to.  Only the animated boolean is required and the arrow direction is set as None automatically.