[Feature] Force loading next page open thumbnails

[cafrew]

Please see below:smile:

[cafrew]

Current state: when "force loading next page" is enabled, and you press next before the page has finished loading, instead of showing the unfinished loading image, it shows the previous image that you are watching.
Suggests: Let's show the thumbnail from the manga page image list, and replace it with full-sized image when loading is finished.

ps: If you find it a great deal of work to adapt for all the sites this script supports, you can make it for e-hentai.org first. Many thanks:smile:

ps 2:To make it simpler, it's not necessarily to fetch the thumbnails. Showing a grey screen with a page number in the middle is sufficient. It's all for fluidness design while reading a manga.