[enh] add show_date setting, to be able to order galleries

[julienmalik]

without showing the date

This allows (by chance) date keys to have non-date values, they just need to be comparable.

[Psycojoker]

I don't understand how this PR is suposed to allow "to be able to order galleries" :/

[julienmalik]

if we don't set date for galleries, I understand the order of the galleries becomes undefined : all dates are implicitely set to "" and then we reversed(sorted()) on this parameter.

if we set a date, but set show_date to false, no dates are rendered, but the date parameter is still used to sort.

[Psycojoker]

Oh, indeed, this isn't very great, I'm going to make a "options["date"] or options["fold_name"]" or something like that to make it deterministic.