Filter effect

Hi!
I think filter effect is done. Clicks and ticks are removed, thanks to @ywwg's crossfade function.

You can listen how it sounds here: http://www.youtube.com/watch?v=nVnN0z2OXhA But in video I've used flanger's controls for changing depth end enabling effect. In this commit, I made separate controls for each decks (Flanger is still works too).

For testing you need to add knob and push button in the skin, or just download and replace "res/skins/LateNight1366x768-WXGA/skin.xml" from this commit https://github.com/xorik/mixxx/commit/4950363ca371b7c54d306c9193570e7af32acbac.

What need to do:
1) Check how it works on any controller (I think there are no problems here, I just can't test at this moment)
~~2) I don't know why, but tooltips are not worked~~
3) Also big problem is hardcoded 44100Hz in filters, but I think it is easy to fix (just replace with real sample rate). https://bugs.launchpad.net/mixxx/+bug/1208816
4) This filter has small resonance effect (which easy to remove, but it sounds good), but it is unable to change resonance force without patching and recompiling filter.

P.S. This effect is VERY useful when mixing: it sounds more smoothly, when you slowly kills sound with filter, not with volume.