Columns should not auto-close

[martinmcclure]

Currently, when the last pane in a column is closed, the column is closed and its space taken by the remaining columns.
This is jarring, even after you get used to it, since it is often a side-effect of an action in a column farther to the left.
A new column coming into existence is more natural, since that is generally a direct effect of something the user has requested.

When a column becomes empty, it should not go away, but become blank, with a largish close icon in the middle to enable manual closing of the column.