You can now set a GTableCellPaintingDelegate per column in GTableView.
For columns with a painting delegate set, the view will defer to the
delegate for painting each cell in that column.
You can now call GTableView::set_size_columns_to_fit_content(true) and
the table columns will grow to fit the content. They will never shrink,
only grow.
This means I can spend a lot less time fidgeting with column widths :^)
I originally thought I'd have to implement text clipping in Painter for
this, but it seems like I can get away without doing that today. :^)
Fixes#390.
