Ticket #3060: live update the panels size when editing layout.

Update the panel split display while adjusting the layout dialog,
because the user should not be expected to know what number of columns
or rows is reasonable without being able to see the effects. Cancel
reverts previous settings.

Signed-off-by: Andrew Borodin <aborodin@vmail.ru>
Make layout dialog left/right arrows more coherent.

Alt+Shift+Left_arrow and Alt+Shift+Right_arrow directly influence the
divider position, but the < and > layout dialog buttons have the
reverse effect, which is unintuitive behavior. Reverse the direction of
< and > influence so that clicking left moves the divider left, and
clicking right moves the divider right.

Signed-off-by: Andrew Borodin <aborodin@vmail.ru>
Merge branch '3060_layout_dialog_enhance'

* 3060_layout_dialog_enhance:
  Make layout dialog left/right arrows more coherent.
  Ticket #3060: live update the panels size when editing layout.