Models: move the declaration of DiveItem to the header

This should allow external use of the type by
including models.h.

Signed-off-by: Lubomir I. Ivanov <neolit123@gmail.com>
Models: add the DiveItem::displayDate() helper

Add this helper so that we can easily wrap the required
memory release after calling get_dive_date_string()
(fixes a small memory leak).

Signed-off-by: Lubomir I. Ivanov <neolit123@gmail.com>
Print: complete columns for the table print

We use the DiveItem struct from models.h so to ease the
display of date, depth and duration. All columns present
in the GTK build are now displayed.

Signed-off-by: Lubomir I. Ivanov <neolit123@gmail.com>
Print: mark some functions in PrintLayout as 'const'

It might be a good practice to declare certain class
'helper' functions as constant members. But I don't
think there are performance benefits to that other
than the readability ones.

Signed-off-by: Lubomir I. Ivanov <neolit123@gmail.com>