UI: render equation columns with separate text fields, rather than padding with spaces

[frothga]

Each container node will hold a set of tab stops collated from its children. When a child renders, it will supply a separate string for each column. EquationTreeCellRenderer will return a widget with separate text fields for each column and a layout manager that puts them in exactly the right horizontal position.

This will give better control over positioning than the current method, which involves padding with narrow spaces until the elements reach approximate alignment. The current approach depends on the available space characters in the chosen font. It is both fragile and expensive. With formal text fields, the horizontal position will be exact.