Reconcile SDRPlay gains with SoapySDR abstractions

[dlaw]

1. Rebase the existing new-gain-controls branch onto latest master
2. Use negative gain values to represent gain reductions. For example, IF gain reduction of 20 to 59 is represented by an IF gain of -20 to -59. RF gain reduction of 0 to N is represented by an RF gain of 0 to -N.
3. Clean up the generic setGain implementation and add a corresponding generic getGainRange.

Let's use this PR to discuss whether this proposal represents the desired semantics for a change. I will be happy to make any additional updates once we come to a consensus.

This PR addresses #10, supersedes #26, and is related to the discussion in https://github.com/cjcliffe/CubicSDR/issues/825.

Cheers, David