add two new options : sortIconUp and sortIconDown

Add sortIconUp and sortIconDown to define the icons that will be used to show the sort state.

I used this in "attach" example.

Signed-off-by: Jean-Yves Beaujean <jybeaujean@webismymind.be>
up.png / down.png

Signed-off-by: Jean-Yves Beaujean <jybeaujean@webismymind.be>