Let fish and fruit regenerate slightly.

This adds minor regeneration to fish (when not gathered) and fruit (always) as a nice extra for casual players.
The values are chosen to be low, as to not affect competitive play much.

Patch by: @Nescio
Differential revision: https://code.wildfiregames.com/D3868
Comments by: @chrstgtr, @marder, @Stan, @wraitii