PsppireDialogAction and subclasses - use function to set class activate and thus...

PsppireDialogAction and subclasses - use function to set class activate and thus minimise deprecation warnings

All subclasses of PsppireDialogAction needed to use GtkActionClass.  This widget however
is deprecated in recent Gtk versions.  Meaning that all instances of PsppireDialogAction
implementations would provoke a deprecation warning at compile time.

This change avoids that by providing a common function to set GtkActionClass->activate.
Thus, there is now only a single warning instead of many.