general: simplify a few unit manipulations

Now that we have defined addition and subtraction on unit
classes, let's use them in a few examples.

Yes, some of these are a bit pointless, because they are
of the kind
        a.mbar - b.mbar => (a-b).mbar

However, these probably should be further simplified
by storing the result in a unit type.

This commit is mostly a proof-of-concept.

Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>

profile-widget/profilewidget2.cpp

@@ -1334,7 +1334,7 @@ void ProfileWidget2::pictureOffsetChanged(dive *dIn, QString filenameIn, offset_
 			auto newPos = std::find_if(pictures.begin(), pictures.end(), [offset, &filename](const PictureEntry &e)
 						   { return std::tie(e.offset.seconds, e.filename) > std::tie(offset.seconds, filename); });
 			// Set new offset
-			oldPos->offset.seconds = offset.seconds;
+			oldPos->offset = offset;
 			updateThumbnailXPos(*oldPos);
 
 			// Move image from old to new position