Ticket #3061: fix [en|dis]abling of layout split adjust buttons.

Switching between vertical and horizontal layouts doesn't properly
activate or deactivate the < and > widget buttons in the layout dialog
according to the setting of the mode being switched to.

Signed-off-by: Andrew Borodin <aborodin@vmail.ru>
Merge branch '3061_layout_split_buttons_fix'

* 3061_layout_split_buttons_fix:
  Ticket #3061: fix [en|dis]abling of layout split adjust buttons.