Remove StartupUri from app.xaml; Add ext methods

No longer need StartupUri due to how the entry point was changed. It
also means that [in order for App.Resources to work] the App.xaml
compile type has been changed to Page.
EntryPoint.cs supports more cmd line args

-noWindow (unchanged)
-jpg / -jpeg / -bmp / -png / -tiff
-screen / -window / -desktop
Changes to ScreenCapturer.cs

Added CaptureMode.Desktop - captures entire desktop from VirtualScreen
information.
Add options to UI

MainWindow now allows for all variations (except Window due to inherent
conflicts with what window  it would capture)