Amend divetrip model to use int units

Amend the DiveItem class to avoid float in favour of int. Add getters
which return display friendly QStrings reflecting user preferences for
(e.g.) depth.

Modify DiveTripModel to support controlled alignment by column; right
align for depth and duration.

Fix problems with utf8 encoding on rating stars, degree symbols and
O2 subscript.

Signed-off-by: Amit Chaudhuri <amit.k.chaudhuri@gmail.com>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Fix column headings for divelist

This looks better and is consistent with the Gtk version.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Correctly format depth and time

We really need those leading 0s.

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