This change also repairs a defect in showing potential LoTW users when
the "Settings->General->Show DXCC, grid, and worked before status"
option is not checked.
Add "Settings->Colors->Decode Highlighting" context menu buttons to
unset b/g and f/g colours. Add colour value as #rrggbb or unset in
text to list items. Improved colour assignment to decodes giving
better and more intuitive behaviour.
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.
