UIView can add C4Control as subview

Overrides addSubview, to provide the ability for adding control objects
to UIViews. This will make the integration of C4 with other basic UIKit
objects easier (i.e. without having to specifically add the
control.view property to any other view).
UIView can add controls directly.
Adding import of new category to C4.h
Adding functionality to C4Control

Can now toggle user interaction of its view, and can also remove the
control from any other view.

NOTE: this might not be an elegant solution, because we are now
clashing between view / control behaviour a bit. But, to be able to add
a control, without being able to remove it is not proper either.

Also, to add the method removeFromSupercontrol is also an awkward clash.