sample: Make the CustomScrollableWidget sample more elaborate
sample: Make the CustomScrollableWidget sample more elaborate

Make the custom widget a generic class, and add and use a derived class.
Also add a second instance of the custom widget, to better test the
interface properties support.

akrisiun/gtk-sharp,orion75/gtk-sharp,Gankov/gtk-sharp,openmedicus/gtk-sharp,openmedicus/gtk-sharp,sillsdev/gtk-sharp,openmedicus/gtk-sharp,orion75/gtk-sharp,antoniusriha/gtk-sharp,openmedicus/gtk-sharp,akrisiun/gtk-sharp,Gankov/gtk-sharp,orion75/gtk-sharp,openmedicus/gtk-sharp,Gankov/gtk-sharp,openmedicus/gtk-sharp,akrisiun/gtk-sharp,akrisiun/gtk-sharp,antoniusriha/gtk-sharp,akrisiun/gtk-sharp,antoniusriha/gtk-sharp,orion75/gtk-sharp,antoniusriha/gtk-sharp,openmedicus/gtk-sharp,Gankov/gtk-sharp,sillsdev/gtk-sharp,sillsdev/gtk-sharp,sillsdev/gtk-sharp,Gankov/gtk-sharp,orion75/gtk-sharp,Gankov/gtk-sharp,sillsdev/gtk-sharp,antoniusriha/gtk-sharp