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/HelpTextWindow.cpp

@@ -0,0 +1,29 @@
+#include "HelpTextWindow.hpp"
+
+#include <QApplication>
+#include <QString>
+#include <QPalette>
+#include <QFile>
+#include <QTextStream>
+
+#include "qt_helpers.hpp"
+#include "widgets/MessageBox.hpp"
+
+HelpTextWindow::HelpTextWindow (QString const& title, QString const& file_name, QFont const& font, QWidget * parent)
+  : QLabel {parent, Qt::WindowCloseButtonHint | Qt::WindowMinimizeButtonHint}
+{
+  QFile source {file_name};
+  if (!source.open (QIODevice::ReadOnly | QIODevice::Text))
+    {
+      MessageBox::warning_message (this, tr ("Help file error")
+                                   , tr ("Cannot open \"%1\" for reading").arg (source.fileName ())
+                                   , tr ("Error: %1").arg (source.errorString ()));
+      return;
+    }
+  setText (QTextStream {&source}.readAll ());
+  setWindowTitle(QApplication::applicationName () + " - " + title);
+  setMargin (10);
+  setBackgroundRole (QPalette::Base);
+  setAutoFillBackground (true);
+  setStyleSheet (font_as_stylesheet (font));
+}