Allow custom columns in table next to Gantt chart

[Chris Nelson]

It would be nice to be able to include Trac-specific columns in the table to the left of the chart. For example, to show status or priority.

This requires a fair bit of JavaScript work. The high-level chart object would need an "addColumn" or something method. Ideally, the existing column handling would be reworked so that the table was empty and the default case called "addColumn('res')", etc.

Then the when adding a task, you'd need to be able to provide values for custom columns (perhaps by having optional argument at the end with a value like "column=value,column=value"?).

Once that infrastructure was in place, the macro needs new options to allow the user to specify which Trac fields to include. Perhaps something like "columns=priority|status".