Revamp the structur of project database

sources/diagramcontext.cpp

@@ -35,6 +35,14 @@ void DiagramContext::add(DiagramContext other)
 	}
 }
 
+/**
+ * @brief DiagramContext::remove
+ * @param key
+ */
+void DiagramContext::remove(const QString &key) {
+	m_content.remove(key);
+}
+
 /**
 	@return a list containing all the keys in the context object.
 */