[Tagging] add leisure=swimming_pool to the core-features

[M∡rtin Koppenhoefer]

I found that leisure=swimming_pool is used almost 8000 times in the
planet (according to taginfo).

There is an old proposal:
http://wiki.openstreetmap.org/wiki/Proposed_features/Swimming_pool

but nobody ever cared to put it on the main feature list. I'd say we
can do it. (Btw.: it used to be a wiki, but unfortunately there was a
technical hurdle introduced so I am no more able to add tags to these
lists). Can someone add this please?
http://wiki.openstreetmap.org/wiki/Key:leisure

IMHO it should also be added to JOSM presets.

cheers,
Martin