The qthelper get_depth_string does now what is expected, depending on
the given bool showdecimal (<20m exception removed).
A preference (units.show_mdecimal_table) was introduced to change whether
decimals are displayed for (metric) max depths in the dive list.
the setting can be change via UI at preferences/units.

Signed-off-by: Hannes Krueger <hanneskrueger@gmail.com>

Signed-off-by: Hannes Krueger <Hannes.Krueger@gmail.com>

Make all calls to get_depth_string use explicit definition regarding
showdecimals. For on-screen usage (especially dive list and profile) use
the user-setting via prefs.units.show_mdecimal. For statistics, details,
etc. decimals are used.

Signed-off-by: Hannes Krueger <Hannes.Krueger@gmail.com>