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/DoubleClickablePushButton.hpp

@@ -0,0 +1,27 @@
+#ifndef DOUBLE_CLICKABLE_PUSH_BUTTON_HPP_
+#define DOUBLE_CLICKABLE_PUSH_BUTTON_HPP_
+
+#include <QPushButton>
+
+//
+// DoubleClickablePushButton - QPushButton that emits a mouse double
+//                 click signal
+//
+//  Clients  should be  aware of  the QWidget::mouseDoubleClickEvent()
+//  notes about receipt of mouse press and mouse release events.
+//
+class DoubleClickablePushButton
+  : public QPushButton
+{
+  Q_OBJECT
+
+public:
+  DoubleClickablePushButton (QWidget * = nullptr);
+
+  Q_SIGNAL void doubleClicked ();
+
+protected:
+  void mouseDoubleClickEvent (QMouseEvent *) override;
+};
+
+#endif