Added Hidden Parameters to vst2

They can be declared from the constructor, the DAW doesn't see them, and they can't be automated
they are saved in the state/preset just like normal params
their indexes are positive and can be obtained with GetParam()
They can be used with IControls just like normal params

Also added SetIgnorePreset() to IParam
if it's enabled for a given parameter - that parameter will reject loading its value from preset/state