Additional quick action buttons when mouse is over items, in Stage

slides sorter.

Stage slides sorter has +/- buttons that appear when you move the mouse
over an item to allow you select or deselect multiple items without
using the keyboard.  This patch add extra actions (remove/duplicate
slide and start slideshow from here) like gwenview, digikam quick actions.

REVIEW: 103538
GUI: Hover buttons for quick actions in Stage Slide Sorter