Add notes about classes that will hopefully be dropped in KF6

Mainly so that no one spends time porting them for KF6...etc

src/declarativeimports/core/datamodel.h

@@ -16,6 +16,8 @@
 
 class QTimer;
 
+// All classes here will hopefully be removed in KF6 (along with DataEngines in general)
+
 namespace Plasma
 {
 class DataSource;

src/declarativeimports/core/datasource.h

@@ -21,6 +21,8 @@
 
 class QQmlPropertyMap;
 
+// This class will hopefully be removed in KF6 (along with DataEngines in general)
+
 namespace Plasma
 {
 class DataEngine;