Enable Qt deprecation warnings
Port away from qVariant{Value,CanConvert}
Port away from qFindChild
DUChainDumper: Improve out

* Only print '==import==>' for real imported contexts
* Don't print declarations twice to begin with
* Be sure we can actually reuse a dumper instance
DUChainDumper: Get somewhat meaningful indentation
Minor: Remove dead code
DUChainDumper: Also print URLs
Introduce Definitions::dump

This reveals another issue in kdev-clang:
Function definitions' ids are not fully qualified. Hence
FunctionDefinition::definition fails when looking up an identifier in a
namespace.
JSON tests: Don't crash suite on null pointers