QmlOutline: Don't use LookupContext due to performance issues

Creating a LookupContext can be sloooow for large projects. We create
one instance for every update in the Outline to get the right icons.

Take a shortcut here and just use the element name directly, ignoring
packages names etc.

This is a hot fix for 2.1, a following patch will change the Icon
retrieval API accordingly.

Reviewed-by: Roberto Raggi
Task-number: QTCREATORBUG-2859