Made FileOpenPicker more generic

FileOpenPicker only worked with MainPage and the class that implemented
IFileOpenPickerContinuable had to be a Page.

Did work to make it more generic so any class that implements an
IFileOpen interface found in ContinuationManager can save a reference to
themselves to App.xaml.cs and be used during app reactivation when
arriving back from the Picker.
Merge branch 'master' of https://github.com/jm991/HeliosApp

Conflicts:
	Helios/Helios/App.xaml.cs
	Helios/Helios/Common/ContinuationManager.cs
	Helios/Helios/ViewModel/MainViewModel.cs
IContinuable and fixed Snap bug

"Snap" orientation doesn't exist on the phone, so I had to remove it
from the MVVM code and from the VisualStateGroups on the pages.

IContinuable interface to refer to any of the Picker interfaces more
generically.