Standardize on ImageGenerator names, fix mini thumbnail display on owner group view.

Standardize on ImageGenerator names, fix mini thumbnail display on owner group view.

To allow for easier theming, ImageGenerator classes for thumbnails are now named as:

  Ansel_ImageGenerator_{type}Thumb

and if there is a matching stack generator, it should be named as:

  Ansel_ImageGenerator_{type}ThumbStack

other types of generators, such as the screen, and mini generators, are named as before:

  Ansel_ImageGenerator_Screen, Ansel_ImageGenerator_Mini etc...