Improve display & formatting of RDS data

[argilo]

Here I have pulled in a number of improvements that were recently made in gr-rds:

• Reset the RDS parser when the frequency is changed bastibl/gr-rds#81
• Remove references to offset word E bastibl/gr-rds#85
• Parser improvements bastibl/gr-rds#86
• Convert Static Variables to Instance Variables bastibl/gr-rds#89
• Fix parsing of "diversion advice" bit bastibl/gr-rds#90
• Fix compiler warnings bastibl/gr-rds#91

The most noticeable improvement is that Station Name, Radio Text, and Alt. Frequencies are updated atomically once all data is received. This prevents the UI from behaving erratically and displaying a mixture of old & new data.

I also changed the RDS panel so that it clears all fields when the Program ID changes (for instance, because a new station has been tuned).