Retain 1 vs. 2 column editing settings independently

[jeffinsf]

The ability to select a one- or two-column editing screen for pages/posts is a very nice enhancement. However, if one changes from two-column to one-column mode (or vice versa), the location of the various widgets do not seem to be restored. In my case, I edit both from a "normal" size screen as well as a mobile device with an 800x480 screen, which has me switching relatively often, necessitating rearranging all the widgets each time. On my mobile device, dragging is not an easy action.

The user experience could be improved in a couple ways:

Request 1) Retain widget locations independently for one- and two-column edit screen

By retaining the locations for the widgets independently, if a user switches from two- to one-column mode and then back again, the widget locations would be retained.

Request 2) Retain one- vs. two-column edit mode setting in user cookie

At a much lower priority, retaining the setting in the cookie on the machine would "automatically" set the mode in cases where the user works in one mode on one machine, and a different mode on another machine.

[TECannon]

Related: #13476

[ocean90]

Related: #18337

[azaozz]

Looking at both this ticket and #18337, seems the best solution wouldn't be to move postboxes when switching columns but to move the columns (the containers) themselves. That would also let us have an "auto" setting when the same user would see two columns on a wider screen and one on narrower. Same for the dashboard: can make it auto-switch depending on the screen width.

[azaozz]

This is handled as part of #18198