ValuePlug : Emit `plugSet()` for all ancestors, not just ValuePlugs
ValuePlug : Emit `plugSet()` for all ancestors, not just ValuePlugs

This is consistent with the documentation for `ValuePlug::emitPlugSet()` and the signature for `Node::plugSetSignal()`.

The immediate motivation for this change is to fix test failures in GafferCortexTest.ParameterisedHolderTest when we move to using Plug rather than CompoundPlug to represent CompoundParameters. Without it, `parameterChanged()` is not called for compound parameters.

ivanimanishi/gaffer,andrewkaufman/gaffer,andrewkaufman/gaffer,ivanimanishi/gaffer,ImageEngine/gaffer,hradec/gaffer,GafferHQ/gaffer,andrewkaufman/gaffer,hradec/gaffer,andrewkaufman/gaffer,ivanimanishi/gaffer,boberfly/gaffer,ImageEngine/gaffer,ImageEngine/gaffer,GafferHQ/gaffer,boberfly/gaffer,lucienfostier/gaffer,lucienfostier/gaffer,hradec/gaffer,lucienfostier/gaffer,johnhaddon/gaffer,johnhaddon/gaffer,andrewkaufman/gaffer,hradec/gaffer,boberfly/gaffer,hradec/gaffer,boberfly/gaffer,GafferHQ/gaffer,appleseedhq/gaffer,johnhaddon/gaffer,appleseedhq/gaffer,johnhaddon/gaffer,ImageEngine/gaffer,GafferHQ/gaffer,lucienfostier/gaffer,ivanimanishi/gaffer,johnhaddon/gaffer,GafferHQ/gaffer,appleseedhq/gaffer,appleseedhq/gaffer