Start implememntation of database based Fox log model and a new Fox log window widget

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.

validators/CallsignValidator.cpp

@@ -0,0 +1,17 @@
+#include "CallsignValidator.hpp"
+
+CallsignValidator::CallsignValidator (QObject * parent, bool allow_compound)
+  : QValidator {parent}
+  , re_ {allow_compound ? R"(^[A-Za-z0-9/]+$)" : R"(^[A-Za-z0-9]+$)"}
+{
+}
+
+auto CallsignValidator::validate (QString& input, int& pos) const -> State
+{
+  auto match = re_.match (input, 0, QRegularExpression::PartialPreferCompleteMatch);
+  input = input.toUpper ();
+  if (match.hasMatch ()) return Acceptable;
+  if (!input.size () || match.hasPartialMatch ()) return Intermediate;
+  pos = input.size ();
+  return Invalid;
+}