CBZ/CBR file type thumbnail support

[maraskan_user]

Related to the bug report about cbz, i collected the things that might be neccessairy to get XnView to handle CBZ/CBR files properly. I guess it's low priority, but in case it's ever tackled, maybe this list makes it easier to find the points that need addressing.

- Even though their internal structure is that of an archive, cbz/cbr files would need to open in the associated program/exernal viewer via double click, unlike normal archive types. Thats because they don't need to be browsed from withing XnView, since they are handled by dedicated viewers.

- cbz/cbr files would need to be known by XnView to be archives, so that the first image is actually decompressed for a thumnail creation. Right now, XnView displays a thumbnail only by accident: if zip file format was used, but no compression applied when creating the cbz.

- Unlike normal archives, the thumbnail should rather be like with normal images, not as with folder/archives. That means, only 1 image, and no folder icon in the background. The cbz/cbr format stores comic books, which are identifiable mainly by their cover.

- XnView would need to also look in subfolders for cbz/cbr files and display their thumbnails as folder overlays, like it does with images.

Greetings
maraskan_user