Rework global timestamp source system

Builds/VisualStudio2013/open-ephys.vcxproj

Builds/VisualStudio2013/open-ephys.vcxproj.filters

Source/CoreServices.cpp

Source/CoreServices.h

Source/Processors/GenericProcessor/GenericProcessor.cpp

Source/Processors/GenericProcessor/GenericProcessor.h

Source/Processors/MessageCenter/MessageCenter.cpp

Source/Processors/MessageCenter/MessageCenter.h

Source/Processors/MessageCenter/MessageCenterEditor.cpp

Source/Processors/MessageCenter/MessageCenterEditor.h

Source/Processors/ProcessorGraph/ProcessorGraph.cpp

Source/Processors/ProcessorGraph/ProcessorGraph.h

Source/Processors/RecordNode/EngineConfigWindow.cpp

Source/Processors/RecordNode/EngineConfigWindow.h

Source/UI/EditorViewport.cpp

Source/UI/SignalChainManager.cpp

Source/UI/SignalChainManager.h

Source/UI/TimestampSourceSelection.cpp

Source/UI/TimestampSourceSelection.h

Source/UI/UIComponent.cpp