Compare QIcon QVariants by their cache key

This makes it so that two QVariants created from the same
QIcon (or from QIcons that were assigned one from eachother)
compare to true.
Unfortunately creating two QIcons with the same path and
comparing them still gives false as they have different cacheKeys

Change-Id: Iafe2bc4082a830f9c6469f083c26a7abbe4b35c5
Reviewed-by: Jędrzej Nowacki <jedrzej.nowacki@digia.com>