Cocoa: emit currentFontChanged signal in font dialog helper

* changeFont is a delegate method of NSFontManager.
* exec() will not be called in Qt Quick.

Task-number: QTBUG-32450
Change-Id: I05e43ce84d28ff32c88d75bd6ee60653de6ca4f0
Reviewed-by: Shawn Rutledge <shawn.rutledge@digia.com>