Added Project-related events, ProjectExplorer now notified of new
Projects
ProjectExplorer now posts an event when any ProjectData is
double-clicked.
Started adding IProjectDataContents tree and IFileImporter tree.
Added ParameterCheck utility class and cleaned up TextData and Contents.
Added extension point for IFileImporter. PlaintextImporter should be
changed to be an extension.
Working on establishing the in-memory representation of an
AnnotationSet.