Add option to keep transparency when converting png images to lossy webp

[abadyr]

It would be a checkbox in the parameters of the webp compression format.

If XnView MP uses cwebp to convert images into webp, it could work for example adding the command "-alpha_q 100" or "-alpha_method int 0".

So a complete command would be something like that, plus any additional chosen options, again assuming MP uses cwebp:

cwebp input.png -q 90 -alpha_q 100 -m 6 -o webp_alpha.webp