DarkThumbs V1.6

A minor release, to fix some small issues and add some small features.

1. FB2 / Mobi options are now dis-entangled (Issue #59)
2. Support SOLID .rar archives (Issue #30)
3. Remove the context-sensitive help support from the manager (Issue #28)
4. For ZIP / CBZ / EPUB, use the latest Windows graphics for thumbnails [supports more image formats if the necessary windows codecs are installed].