Two new environment variables to control special testing behaviour:
* WSJT_TX_BOTH - set to "1" to force transmission on both periods.
* WSJT_REVERSE_DOPPLER - set to "1" to transpose Tx and Rx Doppler
corrections. Use this to test Doppler tracking on a terrestrial
link.
This change incorporates a reorganization of the GUI code with
widgets, validators, models, and item delegates being moved to
sub-directories.
Relax the requirements of the ForeignKeyDelegate and related
CandidateKeyFilter classes to allow them to work with constant model
pointers for both referenced and referencing models.