Made LabelPlugValueWidget disabled when the plug isn't editable.
Added PlugWidget.labelPlugValueWidget() method.
Added NodeUI.plugValueWidget() method.

This allows client code to introspect a NodeUI, and get access to specific PlugValueWidgets within it.
Implemented SectionedCompoundPlugValueWidget.childPlugValueWidget().

This allows the sections to be introspected by StandardNodeUI.plugValueWidget().
Implemented RenderMan shader parameter activation via annotation expressions.

An activator is defined by a global annotation of this form :

	pragma annotation "activator.name.expression" "pythonExpression"

The python expression may reference current parameter values by name, and also use the connected( "parameterName" ) function to query whether or not a parameter is connected.

Activators are then assigned to specific parameters using annotations of this form :

	pragma annotation "parameterName.activator" "name"
Implemented annotation-based uis for RenderManLight node.

This just reuses the RenderManShaderUI, adding registrations for the RenderManLight node too. At some point we still need to better define the relationship between shaders and lights, in particular with respect to how coshaders can be plugged into lights.
Merge pull request #256 from johnhaddon/shaderParameterEnablers

RenderManShader parameter enablers