Implementing the questions "read list" in the roaming settings. This list is synched and questions in this list are removed from all devices.
* Splitting SettingsManager.Save into SaveRoaming and SaveLocal, so saving local data does not trigger a data-chnaged event in other devices.
* Refactoring QuestionsManager.RemoveQuestion and QuestionsManager.RemoveQuestionsAndSave.
* Now, unload question is only needed when a background task completes.
* Remove question in the read-questions-list when synched data changes.
* A collaction cannot be modified whhile iterating though it.
The read-questions-list must be in separate file. It is too big to be in the ApplicationData. ApplicationData values are limited to 8 KB.
We do not need to keep our own implementation of settings version. ApplicationData already has a Version property.