Rigctl support
This contribution brings rigctl support to control some parameters (frequency, current demod used) of the SDRangel instance. More interestingly this has been developed as a plugin and fosters the idea to have extra functionality implemented as plugins that can be optionally instantiated a bit like channel plugins. This is a first go at such a "feature" plugin and will later become more integrated in the global plugins management so some more changes are expected. For now this is an addition at the bottom of the "Preferences" menu list and the interface is detailed here