SlideSorter: insert new slide on double-click.

Previously inserting a new slide required using a context menu
(via right-click) or the main menu-bar. It is now possible
to insert a blank slide by double-clicking in an empty area
of the slide sorter.

Change-Id: Id7ac5f9d3befd02d0a89ad5e1631885c0493ad3d
Reviewed-on: https://gerrit.libreoffice.org/6442
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>