QmlDesigner.PropertyEditor: Use invalid QVariant as default value

Returning a proper default value did confuse the frontend for className.

Task-number: QTCREATORBUG-11682
Task-number: QTCREATORBUG-11683
Task-number: QTCREATORBUG-11682

Change-Id: I9cb0df893597ad2b372935a05a7a5878708ddcda
Reviewed-by: Marco Bubke <marco.bubke@digia.com>
Reviewed-by: Thomas Hartmann <Thomas.Hartmann@digia.com>
QmlDesigner.PropertyEditor: expose ColorLogic

Change-Id: Ibc15d3b05c518dcf71a0b2c014b3076f42e71cdf
Reviewed-by: Marco Bubke <marco.bubke@digia.com>
Reviewed-by: Thomas Hartmann <Thomas.Hartmann@digia.com>
QmlDesigner.Model: Adding QTC_ASSERT to selection

ModelNodes not in the hierarchy should never be selected.
Note: This forces the node to be valid in isInHierarchy().

Change-Id: Ibda7b69d03344232a776d5307788871e4a75fbad
Reviewed-by: Marco Bubke <marco.bubke@digia.com>