WIP: Toggle help view with same decoration rules as parent window

This is only my second ever coding session in Vala so I understand if this is once again not quite usable.

However, still thought it worth submitting now in order to spur thoughts/discussion on how to do it correctly.

It compiles fine but when toggle_help_view is triggered we get the following failures:

(showdown:392822): Gtk-CRITICAL **: 19:39:42.854: gtk_widget_get_visible: assertion 'GTK_IS_WIDGET (widget)' failed

(showdown:392822): Gtk-CRITICAL **: 19:39:42.854: gtk_widget_set_visible: assertion 'GTK_IS_WIDGET (widget)' failed