Fix the zoom-panning for the new profile

This works in a different way compared to the old widget.
To make it work we use vieport()'s height() and width()
and simplify the scroll position to:
scrollPosition = (mousePosition / totalLength) * scrollMaximum

Signed-off-by: Lubomir I. Ivanov <neolit123@gmail.com>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
User manual: replace some images, minor updates

Except for the images involving the dive profile, all the images now are
from Gnome 3. Images for dive profile awaits finalisation of the new dive
profile.

Small changes to the text of the manual have been done, the most important
of which is that dealing with using the unified import dialogue of
Subsurface.

Signed-off-by: Willem Ferguson <willem@willem-Precision-M4700.(none)>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>