QmlDesigner: Remove instanceForModelNode functions

Use directly the NodeInstanceView

Change-Id: I8e4da4fa9e130fe62db178df6a0b60e9bda01a86
Reviewed-by: Thomas Hartmann <Thomas.Hartmann@digia.com>
QmlDesigner: Rename instanceForNode in instanceForModelNode

Change-Id: I182d5b557f5770816059897e746709d054ecac51
Reviewed-by: Thomas Hartmann <Thomas.Hartmann@digia.com>