Picture handling: change data structure to store offset instead timestamp

It makes no sense to store a 64bit time stamp with every picture. Even the
32bit offset (in seconds) from the dive start is WAY overkill. But
switching to that makes the code much more simple in a number of spots.
And makes what is saved to the XML file easier to read, too.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Picture handling: parse XML data

Using XML data files we can now save picture data and load it back in
again. The corresponding code for save-git and load-git is still missing.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Pick the correct API function name for Qt5

Subtle change in function name from Qt4 to Qt5

setAcceptsHoverEvents -> setAcceptHoverEvents

Now Subsurface builds with Qt5 again.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>