Add shortcut to cycle opening column in dual page mode
Add command cycle_first_column
(default binding D
) to cycle the column in which the opening page is displayed in multi-column layout.
The preliminary commit adds functions to simplify manipulating the first-page-column
config string by converting it to and from integer arrays.
Fixes: #191 (closed), #234 (closed), #257 (closed) (and to some extent #289)