PlugValueWidget : Improve handling of read-only metadata
PlugValueWidget : Improve handling of read-only metadata

Don't assume we know what the metadata is named, and correctly update when an ancestor node is made read only.

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