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.

widgets/FrequencyLineEdit.cpp

@@ -0,0 +1,71 @@
+#include "FrequencyLineEdit.hpp"
+
+#include <limits>
+
+#include <QDoubleValidator>
+#include <QString>
+#include <QLocale>
+
+#include "moc_FrequencyLineEdit.cpp"
+
+namespace
+{
+  class MHzValidator
+    : public QDoubleValidator
+  {
+  public:
+    MHzValidator (double bottom, double top, QObject * parent = nullptr)
+      : QDoubleValidator {bottom, top, 6, parent}
+    {
+    }
+
+    State validate (QString& input, int& pos) const override
+    {
+      State result = QDoubleValidator::validate (input, pos);
+      if (Acceptable == result)
+        {
+          bool ok;
+          (void)QLocale {}.toDouble (input, &ok);
+          if (!ok)
+            {
+              result = Intermediate;
+            }
+        }
+      return result;
+    }
+  };
+}
+
+FrequencyLineEdit::FrequencyLineEdit (QWidget * parent)
+  : QLineEdit (parent)
+{
+  setValidator (new MHzValidator {0., std::numeric_limits<Radio::Frequency>::max () / 10.e6, this});
+}
+
+auto FrequencyLineEdit::frequency () const -> Frequency
+{
+  return Radio::frequency (text (), 6);
+}
+
+void FrequencyLineEdit::frequency (Frequency f)
+{
+  setText (Radio::frequency_MHz_string (f));
+}
+
+
+FrequencyDeltaLineEdit::FrequencyDeltaLineEdit (QWidget * parent)
+  : QLineEdit (parent)
+{
+  setValidator (new MHzValidator {-std::numeric_limits<FrequencyDelta>::max () / 10.e6,
+        std::numeric_limits<FrequencyDelta>::max () / 10.e6, this});
+}
+
+auto FrequencyDeltaLineEdit::frequency_delta () const -> FrequencyDelta
+{
+  return Radio::frequency_delta (text (), 6);
+}
+
+void FrequencyDeltaLineEdit::frequency_delta (FrequencyDelta d)
+{
+  setText (Radio::frequency_MHz_string (d));
+}