Improving columnChooser, fixing overflow-y of hdiv

Hello Tony,

I improved `columnChooser` one more time. Additionally I included small fix

    .ui-jqgrid-hdiv { overflow-y: hidden; }

in `ui.jqgrid.css` which removes unneeded vertical scroll bar from the header of the last column in Google Chrome.

Best regards
Oleg
P.S. I hope that you publish soon the new release of jqGrid.