Allow selecting/moving Slices whose boundaries are contained in another Slice

[Gaspar Capello]

You cannot select slices within the bounds of larger slices. You have to move the larger slices out of the way before you can move the nested slice (selecting via the drop down menu doesn't help because as soon as you click the selected slice to move the outer slice takes over/priority).

Internal report on ticket: https://igarastudio.zendesk.com/agent/tickets/6001

Perhaps it could be added as an item to the issue: #1651