WIP: Added LazyConditionWalker, for easy condition AST manipulation

This class allows the user to walk ("visit") over the 'AST' of a
LazyCondition using a visitor/handler based approach, where one
can register a handler method to specific parts of the tree, allowing
manipulation or using it as a way to quickly verify it.
Callback: Implicit cast from QVariant to other types

This small patch enables Callback to "unpack" a passed QVariant,
enabling the conversion mechanisms work as expected.