re-open the video artwork selection dialog after having chosen a specific artwork

This is an improvement for the artwork selection process. Currently a user opens the video info dialog for a video, then selects "Choose art" which opens a dialog with a list of all the different artwork types and their current image. Then the user chooses which artwork type to change which closes the "Choose art" dialog and opens the file browser which lets the user choose a specific image. After either choosing an image or closing the dialog we go back to the video info dialog and not to the "Choose art" dialog even though that's the previously active dialog. This has annoyed me several times, either when I accidentally chose the wrong artwork type and had to go back or when I wanted to change multiple artwork types. In either case I had to press "Choose art" again in the video info dialog to get back into the artwork selection dialog.

The diff looks bigger than it is (I had to refactor a bit because there were some early returns). Basically what I have changed is to call OnGetArt() again after the user has chosen a specific image. The user can still close that dialog by executing the Back action.

Obviously this is nice to have and therefore for Frodo+1.