[enhancement] Index of images in title bar, etc.

[qinxs]

1. Display image index in title bar, e.g. [1/20] name.jpg.
2. optimise the transparent background display of png, gif images, like this:
   

Thanks for developing such a small and high performance image viewer!

[sylikc]

There was a patch somewhere in one of the issues of number 2 which I forgot to merge... and for 1, I suppose making the titlebar configurable would be nice to have, but right now you can use the "filename" area... option FileNameFormat