Polish the image viewer example.

- Open files passed on the command line.
- Point the file dialog to the pictures location
  and use a filter string for the supported types.
- Set the window title according to file name.

Task-number: QTBUG-37203
Task-number: QTBUG-39287
Change-Id: I4e5e43875c3a7544c862c054181e75942939c1d5
Reviewed-by: David Faure <david.faure@kdab.com>