Derive C++ namespaces from each IDL module's location and use those
qualified names when generating binding code.
Also Teach dictionaries their owning IDL module path so dictionary C++
types can be qualified the same way as interfaces. This removes the need
for the generated `using namespace Web::*` hack and the hard-coded
namespace list.
Also fix DOMURL.idl to refer to the IDL interface name `URL`, not the
C++ implementation name `DOMURL`.
This required some changes in LibURL & LibIPC since it has its own
definition of an BlobURLEntry. For now, we don't have a concrete usage
of MediaSource in LibURL so it is defined as an empty struct.
This removes one FIXME in an idl file.
