QmlDesigner.propertyEditor: fix for color editing

In case alpha != 255 the constructor of QColor does not work.
Since we do resolve alias properties in the code model
we can explcitly "cast" to QColor using the property reader.

Same for url.

Change-Id: I15f67592efbf7c574d49cf687353a280cf3120e6
Reviewed-by: Marco Bubke <marco.bubke@digia.com>