Convert 7.4 EMF Tag-Based Extension Model Framework To 7.5 Model Extension Definition Framework

Convert the code encompassing the "ext-custom" extension property prefix to the new model extension framework. A model extension definition file, an Eclipse extension, and a ModelExtensionAssistant will need to be created. Since in 7.4 the extension properties could only be viewed in a dialog, and in the new framework extension properties are shown in the Properties View, changes to or new property sources and descriptors will be needed.