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/DecodeHighlightingListView.cpp

@@ -0,0 +1,77 @@
+#include "DecodeHighlightingListView.hpp"
+
+#include <QAction>
+#include <QColorDialog>
+
+#include "models/DecodeHighlightingModel.hpp"
+#include "MessageBox.hpp"
+
+#include "pimpl_impl.hpp"
+
+class DecodeHighlightingListView::impl final
+{
+public:
+  impl ()
+    : fg_colour_action_ {tr ("&Foreground color ..."), nullptr}
+    , bg_colour_action_ {tr ("&Background color ..."), nullptr}
+    , defaults_action_ {tr ("&Reset this item to defaults"), nullptr}
+  {
+  }
+
+  DecodeHighlightingListView * self_;
+  QAction fg_colour_action_;
+  QAction bg_colour_action_;
+  QAction defaults_action_;
+};
+
+DecodeHighlightingListView::DecodeHighlightingListView (QWidget * parent)
+  : QListView {parent}
+{
+  addAction (&m_->fg_colour_action_);
+  addAction (&m_->bg_colour_action_);
+  addAction (&m_->defaults_action_);
+  connect (&m_->fg_colour_action_, &QAction::triggered, [this] (bool /*checked*/) {
+      auto const& index = currentIndex ();
+      auto colour = QColorDialog::getColor (model ()->data (index, Qt::ForegroundRole).value<QBrush> ().color ()
+                                            , this
+                                            , tr ("Choose %1 Foreground Color")
+                                                .arg (model ()->data (index).toString ()));
+      if (colour.isValid ())
+        {
+          model ()->setData (index, colour, Qt::ForegroundRole);
+        }
+    });
+  connect (&m_->bg_colour_action_, &QAction::triggered, [this] (bool /*checked*/) {
+      auto const& index = currentIndex ();
+      auto colour = QColorDialog::getColor (model ()->data (index, Qt::BackgroundRole).value<QBrush> ().color ()
+                                            , this
+                                            , tr ("Choose %1 Background Color")
+                                                .arg (model ()->data (index).toString ()));
+      if (colour.isValid ())
+        {
+          model ()->setData (index, colour, Qt::BackgroundRole);
+        }
+    });
+  connect (&m_->defaults_action_, &QAction::triggered, [this] (bool /*checked*/) {
+      auto const& index = currentIndex ();
+      model ()->setData (index, model ()->data (index, DecodeHighlightingModel::EnabledDefaultRole).toBool () ? Qt::Checked : Qt::Unchecked, Qt::CheckStateRole);
+      model ()->setData (index, model ()->data (index, DecodeHighlightingModel::ForegroundDefaultRole), Qt::ForegroundRole);
+      model ()->setData (index, model ()->data (index, DecodeHighlightingModel::BackgroundDefaultRole), Qt::BackgroundRole);
+    });
+}
+
+DecodeHighlightingListView::~DecodeHighlightingListView ()
+{
+}
+
+QSize DecodeHighlightingListView::sizeHint () const
+{
+  auto item_height = sizeHintForRow (0);
+  if (item_height >= 0)
+    {
+      // set the height hint to exactly the space required for all the
+      // items
+      return {width (), (model ()->rowCount () * (item_height + 2 * spacing ())) + 2 * frameWidth ()};
+    }
+  return QListView::sizeHint ();
+}