Implements the necessary functionality to allow Document#column_box to work.
x coordinate of the left edge of the current column
Moves to the next column or starts a new page if currently positioned at the rightmost column.
x co-orordinate of the right edge of the current column
The column width, not the width of the whole box. Used to calculate how long a line of text can be.
Column width including the spacer.
[Validate]