core (common): improved flexibility of jid selector: - JidSelector can now display a custom set of jids, using the new `to_show` property, see comments for details. - a new "implicit_update" property can be set to False (it's True by default) to avoid the automatic "update" on instance initiation. If unset, use must call "update" explicitly.

-e git+https://github.com/kivy/kivy.git#egg=kivy
-e hg+https://repos.goffi.org/sat#egg=sat
-e .