Xml(Xaml) code completion improvments

* Most of this changes are so XmlElement, Attribute and AttributeValue CodeCompletion show more ofter e.g.: 
 * After typing "<Stac" and pressing (Ctrl+Space) now code completion displays
* Another change is if user write "<StackLayout Ori" and select Orientation from code completion(and has setting "Automatically insert punctuations" enabled) code completion for values(Horizontal,Vertical) is automatically displayed.
* Also closing quote for attribute value is now SkipChar which means if user after typing value types quote.. Quote is "overwritten" same as for closing brackets in C#....
* Another small change is that XmlCompletionData checks if user setted Icon and if user setted don't override with Xml generic icon.