Improve visuals for column resize

[dennisrd]

Resizing the width of a column is done by dragging the "shaded" triangle in the lower-right corner of the column's header. When the view is enlarged by hitting Ctrl-+, those triangles keep their original size, which makes it hard to spot them.
The triangles should resize along with the view.
Or maybe they can be dropped altogether; in that case, the mouse cursor should change when it is placed over the divider between 2 adjacent column headers, to indicate that the divider can be dragged left or right in order to resize the column to the left of it.