#74 Keep relative reading position

[Ark]

Whenever you use the scroll bars, MComix should remember their respective relative positions. If you change the size of the viewport or if you change the zoom factor, these relative positions should be restored accordingly.

Currently, MComix does not keep the relative positions of the scroll bars:

1. Zoom to 100%. Switch to "Manual zoom" mode. Switch to "Manga mode".
2. Go to an image that is much bigger than the viewport in both directions. You could start reading, since you are at the upper right corner of the image.
3. Switch to "Best fit" mode.
4. Switch back to "Manual zoom" mode again. Now you are at the upper left corner of the image. Thus you have to scroll back to the upper right corner by yourself. This is annoying.