Allow multiple thumbs selection for copy, delete, paste, move

[Esshahn]

It would be great if one could select more than just one thumb

E.g. in this scenario I could hold down shift and click on 6 and Petmate would select 3,4,5,6.

With the selection active, I could copy, paste, delete and move the screens together. This is probably quite difficult to implement, but it would be a life saver when working with multiple screens.