Rename gain controls 'IF' and 'RF' and keep the old 'IFGR' and 'RFGR' for backward compatibility without listing them

[fventuri]

As discussed with @SDRplay and @vsonnier, we should rename the gain controls to 'IF' and 'RF', and keep the old gain reduction 'IFGR' and 'RFGR' for backward compatibility without listing them explicitly in 'listGains()'.

This change is now in the 'new-gain-controls' branch.