Lots of work to get SDR to redirect to plugin when getting and setting settings values.  Same for SDR options dialog.
RTL SDR plugin now working with latest rtl-sdr library.  USB only.

Had to change all local methods in deviceInterface to virtual so we can implement override in sdr.cpp.

The plugins themselves implement deviceInterface and thus have deviceInterface methods and variables

SDR also implements deviceInterface for internal devices.  It delegates to plugin when plugin is active device.  So SDR has to be very careful to manage whether its using SDR device interface methods/variables or plugin-> deviceInterface methods/variables.