#16 optional override of 3rd party viewers

[John-Philip Leonard Johansson]

A ThumbView user called Robin commented that PhotoShop
overrides ThumbView when it comes to generating
thumbnails. This was a issue previously known to me. My
conclusion at that time was that PhotoShop uses some
undocumented approach to generating thumbnails and I
couldn't see how so I let it be.

The request is that when installing (or at any time?)
allow the user to make ThumbView override any installed
applications thumbnail generation. I.e. using
ThumbViews instead of say PhotoShops thumbnails.