[System] Fix a problem with serializing configuration values

* After reading the configuration file we overwrote the serialized
  value in the property getter unless we accessed the deserialized
  value before.

* When setting the deserialized/serialized value we also have to
  change the defaulted flag.

(This is part of upstream PR #964)

Change-Id: I54ec78ff5e9dd7bfe49802941506defa464dd86a
Fix tooltip display for ListView when using Groups

This fixes WS-73 (Xamarin 21346).

Change-Id: I324409d9d5ce72a74361a658dfdf14093c819a2a