Make the notebook portion (dive notes/equipment/info) a scrollable window

Make the notebook portion (dive notes/equipment/info) a scrollable window

This makes things start up with the wrong size, which is somewhat
annoying, but by doing so avoids a bigger annoyance, namely that the
three panes move around when moving between dives.

In particular, if the initial dive didn't have much of an equipment
list, the initial size allocated for the notebook is fairly small and
determined mainly by the size of the the Dive Notes page.  However, when
you then scroll around in the dive list, you might hit a dive with lots
of equipment, and suddenly the panes dividing the different parts of the
subsurface application window will jump around to make room.

That's horribly annoying, and actually makes things like double-clicking
dives in the dive list not work right, because the first click will
select it, and cause the dive to move around (so the second click will
hit a totally different dive).

Now, making the notebook be in a scrollable window means that if the
size of the notebook changes, it might get a scrollbar, but the panes
themselves do not move around.

The initial sizing of that thing being wrong is annoying, though.  We
need to figure out a separate solution to that.

[ Side note: currently it uses GTK_POLICY_NEVER for the horizontal
  scroll-bar, just to avoid the horizontal size also starting out wrong,
  which is *really* nasty.  If we can solve the initial size issue, we
  should make the horizontal scroll-bar be GTK_POLICY_AUTOMATIC too. ]

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>