Problem: A right-click on the 'statuscolumn' does not open the
popupmenu, even if a cell without a clickdef is clicked.
Clicking the %C fold item does not open/close the fold.
Solution: Open the popupmenu when there is no clickdef like right-clicking
the sign/numbercolumn does. Fill "linebuf_vcol" when drawing the
'statuscolumn' to handle foldcolumn item clicks.