QVariant::Handler should be extendable

The QVariant::operator== does not compare custom types, as written in the documentation.

So there is no way to find (QCheckBox::findData) a value again if you've put it into a QCheckBox::itemData. The QVariant::Handler should be extendable to allow the comparison of own defined types when needed.

Better yet if we could detect at the time of the metatype registration if extra operators (like operator==) are defined, using templates. Boost has some code to detect that.