Tabledrag icon incorrectly shows cardinal arrows on vertical sorts.

[laryn]

Description of the bug

I noticed that the tabledrag icon incorrectly shows the cardinal (four direction) arrows even when the allowable drag is only vertical. It would be better if this condition only showed a vertical sort icon. And we should probably convert to SVG while we're at it.

[laryn]

I submitted a PR that:

• Changes the 'sort' (no nesting) drag icon to a vertical sort icon
• Changes the drag icons to SVG

Changed (new) version is on the left: