Add Transverter Offset setting

[K7MDL2]

1. Intercept CI-V frequency messages from the radio being passed through to a PC and substitute the transverter frequency. Same for PC to radio direction.

2. Add a way to use a menu and/or buttons to easily choose a band, transverter, or not. If it is a transverter band then add the appropriate offset. - Done

3. Save and restore frequency for each Xvtr band and the non-Xvtr band they switched from. - Done

[K7MDL2]

Xvtr bands now work. Can press BtnC to cycle through Xvtr OFF and 4 Xvtr. Xvtrs are hard coded in a switch statement today. Need to bring out to a SD card or menu config page. That will be a separate tracking issue under Add Means to Configure

[K7MDL2]

The band/frequency/mode/datamode/filter/AGC/Preamp/Atten are captured prior to turning on XVTR mode, is restored when XVTR mode is turned off. Same for each XVTR band.

[K7MDL2]

The entire bands table (1344 bytes) is saved to SD card file "/bands.data". It is read into memory overwriting the program defaults in app_setup();

It is used primarily to set the Xvtr bands to their last known values for freq, mode, filter, datamode, agc, preamp, attn. The last non-XVTR band used is also saved. Some values like freq are stored for any band, XVTR or not, but the only cases we are interested in for writing to the radio are related to XVTR usage and returning the radio back the way it was.