DecodeSignal: Reset and re-use existing decoder sessions

DecodeSignal: Reset and re-use existing decoder sessions

Instead of destroying and re-creating decoder sessions, re-use
previously created sessions by just terminating their current work and
resetting internal state.

This implementation most proboably is incomplete in respect of changes
to the stack hierarchy or individual decoders' properties. This needs
more attention. When the stack is kept but its configuration has changed,
the involved properties need to get "somehow applied to" the stack's
instances before re-using it with new input data will yield correct results.