QmlDesigner: Refactor QmlItemNode::createQmlItemNode

Change-Id: Ic3f5db354cce4fd992008fa89aa4748254ea290e
Reviewed-by: Thomas Hartmann <Thomas.Hartmann@digia.com>
C++: fix tooltip for constantValue of enumeratorDeclaration

E.g.:
enum E { x = 1 + 2 + 1 };

constantValue for enumeratorDeclaration currently is equal: '1+2+1'
(spaces between are removed).
Fixed version produces "1 + 2 + 1".

Change-Id: I38f7703bd5c856c28aadeecd7f7923aa4a7da422
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@digia.com>