Implement composite inspector

[BatchDrake]

In some scenarios, a digital signal has been modulated twice. This is the case for RDS: it consists of two mirrored BPSK signals modulated at 57 kHz with respect to the FM baseband signal.

Chaining two inspectors together should be straightforward: between every two adjacent inspectors, a spectral tuner selects the desired subchannel. The configuration part is perhaps the most critical one, as the user may want to adjust the parameters for all demodulators from the GUI.