query: allow attribute engine in element define
query: allow attribute engine in element define

The DTD is conservative about certain definitions. Extend define
with attribute engine to provide engine-specific query definitions.

Also, to enable engine-specific definitions, we need to be able
to write multiple defines per query.

Signed-off-by: Jan Viktorin <3eab6ac54c005c4408f608556a6663b7f2abede7@fit.vutbr.cz>

BeeeOn/server,BeeeOn/server,BeeeOn/server,BeeeOn/server