[librender] DisplaySink is observable

src/libRender/RadiumNBR/Gui/NodeGraphControllerGui.cpp

...RadiumNBR/Gui/RenderGraphEditor/RenderGraphEditorView.cpp

...RadiumNBR/Gui/RenderGraphEditor/RenderGraphEditorView.hpp

src/libRender/RadiumNBR/NodeBasedRenderer.cpp

src/libRender/RadiumNBR/NodeBasedRenderer.hpp

...adiumNBR/NodeGraph/PremadeNodes/Sinks/DisplaySinkNode.cpp

...adiumNBR/NodeGraph/PremadeNodes/Sinks/DisplaySinkNode.hpp

src/libRender/RadiumNBR/NodeGraph/RenderGraph.cpp