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.cpp

@@ -0,0 +1,14 @@
+#include "DoubleClickablePushButton.hpp"
+
+#include "moc_DoubleClickablePushButton.cpp"
+
+DoubleClickablePushButton::DoubleClickablePushButton (QWidget * parent)
+  : QPushButton {parent}
+{
+}
+
+void DoubleClickablePushButton::mouseDoubleClickEvent (QMouseEvent * event)
+{
+  Q_EMIT doubleClicked ();
+  QPushButton::mouseDoubleClickEvent (event);
+}