Smart Shuffle Mode

[codeminic]

Requirement from da wife:

When in random mode (this is the only mode supported at the moment), a picture should not be shown more than twice

Two ideas:

• Remove shown pictures from the images that can be shown. Once there are no images left, start again with all available pictures
• Implement a fancy shuffle mode that uses randomization only for the order of the picture, not for picture selection itself. This would enable another feature; manual backward / forward within the shuffled pictures