Added generated thumbnails to the grid view

- Added the functionality to save the generated thumbnails under the
  application's sandbox data/thumbnails folder
- Added functionality to create thumbnails for the newly added video files
  after the next app start
- Added functionality to delete thumbnails for the video files that are
  not exist any more
- Modified the thumbnail size to have a nicer look in the grid view
- Now the thumbnails of each video file are displayed in the grid view