Fixed read-only RenderManShaderUIs.

There were two problems. Firstly, the SectionedCompoundPlugValueWidget wasn't passing on the read-only status to its children. Secondly, the RenderManShaderUI would clobber the read-onlyness with its activators.
Fixed bug whereby read-only PlugValueWidgets were accepting drags.
Merge pull request #513 from johnhaddon/readOnlyNodeEditorFixes

Read only node editor fixes