Added the StartPageView.xaml with a navigation list or NavigationItem.cs.

Updated Caliburn.Micro from 1.5.0 to 1.5.1
Added the 'Settings' screen and fixed the start screen glyphs.
Added the main page and the 'hearing test' view. Tied up the navigation.

Added the IXmlItemsFileManager.cs to facilitate serializing/deserializing
item lists to/from XML file.