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/LiveFrequencyValidator.cpp

@@ -0,0 +1,99 @@
+#include "LiveFrequencyValidator.hpp"
+
+#include <QLocale>
+#include <QString>
+#include <QComboBox>
+#include <QLineEdit>
+
+#include "models/Bands.hpp"
+#include "models/FrequencyList.hpp"
+
+#include "moc_LiveFrequencyValidator.cpp"
+
+LiveFrequencyValidator::LiveFrequencyValidator (QComboBox * combo_box
+                                                , Bands const * bands
+                                                , FrequencyList_v2 const * frequencies
+                                                , Frequency const * nominal_frequency
+                                                , QWidget * parent)
+  : QRegExpValidator {
+      QRegExp {       // frequency in MHz or band
+        bands->data (QModelIndex {}).toString () // out of band string
+          + QString {R"(|((\d{0,6}(\)"}    // or up to 6 digits
+          + QLocale {}.decimalPoint () // (followed by decimal separator
+          + R"(\d{0,2})?)([Mm]{1,2}|([Cc][Mm])))|(\d{0,6}(\)" // followed by up to 2 digits and either 'm' or 'cm' or 'mm' (case insensitive))
+          + QLocale {}.decimalPoint () // or a decimal separator
+          + R"(\d{0,6})?)|(\d{0,3}(\)"        //  followed by up to 6
+                                              //  digits or a decimal number
+          + QLocale {}.decimalPoint () // or a decimal separator
+          + R"(\d{0,6})?[Kk]))"        // followed by a 'k' or 'K'
+      }
+      , parent
+    }
+  , bands_ {bands}
+  , frequencies_ {frequencies}
+  , nominal_frequency_ {nominal_frequency}
+  , combo_box_ {combo_box}
+{
+}
+
+auto LiveFrequencyValidator::validate (QString& input, int& pos) const -> State
+{
+  auto state = QRegExpValidator::validate (input, pos);
+  // by never being Acceptable we force fixup calls on ENTER or
+  // losing focus
+  return Acceptable == state ? Intermediate : state;
+}
+
+void LiveFrequencyValidator::fixup (QString& input) const
+{
+  QRegExpValidator::fixup (input);
+  if (!bands_->oob ().startsWith (input))
+    {
+      if (input.contains ('m', Qt::CaseInsensitive))
+        {
+          input = input.toLower ();
+
+          QVector<QVariant> frequencies;
+          for (auto const& item : frequencies_->frequency_list ())
+            {
+              if (bands_->find (item.frequency_) == input)
+                {
+                  frequencies << item.frequency_;
+                }
+            }
+          if (!frequencies.isEmpty ())
+            {
+              Q_EMIT valid (frequencies.first ().value<Frequency> ());
+            }
+          else
+            {
+              input = QString {};
+            }
+        }
+      else if (input.contains (QChar {'k'}, Qt::CaseInsensitive))
+        {
+          // kHz in current MHz input
+          auto f = Radio::frequency (input.remove (QChar {'k'}, Qt::CaseInsensitive), 3);
+          f += *nominal_frequency_ / 1000000u * 1000000u;
+          input = bands_->find (f);
+          Q_EMIT valid (f);
+        }
+      else
+        {
+          // frequency input
+          auto f = Radio::frequency (input, 6);
+          input = bands_->find (f);
+          Q_EMIT valid (f);
+        }
+
+      if (bands_->oob () == input)
+        {
+          combo_box_->lineEdit ()->setStyleSheet ("QLineEdit {color: yellow; background-color : red;}");
+        }
+      else
+        {
+          combo_box_->lineEdit ()->setStyleSheet ({});
+        }
+      combo_box_->setCurrentText (input);
+    }
+}