Fixed bug in ShaderUI.

When no shader was loaded, string values were being passed to a setEnabled() method which will only accept a bool.
Added support for "help" annotation in RenderManShaderUI.

This provides help for the shader as a whole and is mapped into the MetaData as the node description, appearing in the NodeEditor as a tooltip.

Fixes #536.
Merge pull request #537 from johnhaddon/shaderHelpAnnotation

Shader help annotation