proposed solution for issue #233

Due to model structure ( no List properties, only database properties), sometimes a model is modified but this modification is not "captured" by their properties: the more clear sample is TimeEntryModel / Tags, in a common sense the Tags belong to TimeEntryModels but there is not a List<TagModel> property.

I have added an utility function ( SetTimeEntryAsDirty) to supply the functionality of mark a model as dirty although the properties has been not changed.