Effect enable state

This PR pipes the enable state transitions down to the effect itself.
So the effect can cleanup or init as required.

It offers also an option to bypass the raming from the effect framework at enable transitions.
An effect can set effectRampsFromDry to true and handle the required ramping itself.
This was introduced to the bitcrusher and to the bit perfect EQs

fixes https://bugs.launchpad.net/mixxx/+bug/1377705
Yes, it act differently, because IMHO it should act like that. If you have a 96 k sample rate, a 44100 sample rate is already down-sampling. Thats why the downs amping knob is normalized to 1.
We have the same issue at the filter effects. A the maximum cut-off frequency depends on the samplerate. Thats the reason the cut-off is normalized as well. We may handle a displayed value of these parameter in Hz by a control hint. LV2 has the same approach.
See:
http://lv2plug.in/ns/lv2core/#sampleRate