MainPage clean up

- Moved MainPage.xaml to /Pages/
- Moved Notifications and Friends into MainViewModel
- MainPage is now only binding against one view model
- Added event to client that is raised when a login is required (might
have broken the welcome page)
- Added search button to application bar
- Added notification popup (content still missing)
- Added working notification count
- Renamed App.PostViewModel to App.MainViewModel
- Changed default page to account for the new location
- Changed sample data