WPF: Fixed bugs in LibraryPreferences.

- LibraryPreferences: Fixed several bugs such as library folders not
being shown, Update Library panel now being shown in MainWindow when
clicking on the Update Library button.
- Changed the definition of ILibraryPreferencesView for
OnAddFolder/OnRemoveFolders.
- MainWindow: Can now hide the UpdateLibrary panel.
- Should have fixed build on iOS, Android, Linux and OSX.