Fanart/Poster image selection related cleanup

Simplify dlgImgSelect callers by returning a DialogResult from the ShowDialog overloads.
Expose the selected image via a public Results property. 

This way the local Poster/Fanart variable won't get set to Nothing while we scrape multiple movies and prevent a NULL-deref. 

Additionally, don't display the selection dialog in cases where there is no image(s) to select.