Updates for RTL-SDR, DSD demod and main menus

Changes

RTl-SDR source plugin

New rates: 256k and 1600k

DSD demod plugin

Show color code in DMR specific status area

Main menus

Preferences with only the audio settings has been moved to a main entry in the top menu as Preferences. The Audio device choice dialog has been moved under it. This will allow more general preferences to be specified.

Fixes

DSD demod plugin

Fixed missing initialization in D-Star header process causing random segmentation fault.