client helpers use client properties

[eylles]

changed the _client.turn_off() and _client.turn_on() helpers to use the c.hidden property instead of the tagging/untagging dance in accordance to a comment in #140

so far this seems to work or the swallowing module and i haven't found any particular new corner case

testing for scratchpads and the tabbed module need to be done.

[Nooo37]

Works surprisingly fine for me. Sadly one exception I found are scratchpads where animations break in some cases (some incomming animations I think). The module probably checks for whether the scrachpad is already on the tag, which is true for this PR but false for the current code

[eylles]

hmmmm how about creating 2 sets of helpers, one just for the scratchpads and the other for everything else?

or perhaps have another argument for the helper that is only passed on the scratchpads module?