Add hooks for filtering and fully rewrite rewrite selection requests
and events (what existing XACE hooks cannot do), e.g. for supporting
separate selection name spaces.
The hook can change individual fields in the parameter struct, so
operation continues with these changed values (eg. replace the original
selection name atom by a different one). It's also possible to stop
operations completely (with given result code) - in that case the
hook needs to take care of the remaining work to do (eg. sending events)
Signed-off-by: Enrico Weigelt, metux IT consult <info@metux.net>