Change window swallowing to use rules.

[kodesoul]

Feel free to ignore this PR if it goes against default structures.

This PR replaces the default behaviour of setting swallow rules in the theme with setting them with awful.rules.
I'm not sure if this is better but to me seems to make more sense.

[eylles]

what happens when you turn the swallow filter off with false?, i added that horrible juggling with the filter var because using booleans with lua in the way it is used in here will not act at all in the expected way.

[kodesoul]

I've done some testing and it seems to work just fine. I guess since it's only one switch i.e
window_swallowing_activated toggling works fine. In terms of setting the swallow rules
as client properties it seems that awful is handling any possible funkiness.

[Nooo37]

I really like that idea. Now that you PR it, it seems to be almost obvious that it should be that way. Good job! One thing I don't like though is how one has to set swallow_parent to true for all clients in order to get "the default behavior". I think it should just swallow everything by default and then the user can add exceptions. We used to have dont_swallow(filter list) but that's obviously not a perfect naming scheme either. Maybe swallow_exception would fit better?