QmlDesigner: Removing unused include

Plugins that are based on Qt Quick Designer can break,
because they use ItemLibraryInfo but not qml.

Change-Id: I222225d04ebde99f0687e15177ac71b378e4a108
Reviewed-by: Eike Ziller <eike.ziller@digia.com>
Reviewed-by: Tim Jenssen <tim.jenssen@digia.com>
VariableChooser: Slim down header and implementation

Change-Id: Ia39c94c2e87717c23cb0d7466b68c922b033979b
Reviewed-by: Eike Ziller <eike.ziller@digia.com>