columnview: Handle focus movement properly

columnview: Handle focus movement properly

- grab_focus() on a row (happens with scroll_to()) keeps the focus
  column intact if possible.
- <Tab> and <Shift-Tab> move through the cells in order, and move
  to the next row when at the end.
- <Up> an <Down> move to the next/prev row, keeping the same column
  focused.
- <Left> and <Right> move to the prev/next cell, if one is available.
  If not, they stay where they are.