PR: add a toggle to show units in dive list cells

[Lubomir I. Ivanov]

Stefan has created a PR for adding this functionality:
https://github.com/Subsurface-divelog/subsurface/pull/640#issuecomment-334929737
(^ screenshots of Settings and the dive list included)

i'm bringing this to the mailing list for discussion.
do you think that having the units (e.g. meters) in each cell is a good idea?

lubomir
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