fix issue#739

This fix is for the issue #739 that enableTextSelectionOnCells doesn't work in non-IE browser.

I found the cause is in setFocus method (in slick.grid.js).
If non-IE browser , text-selection is cleared after focus method executed on focusSink element.

I don't know well about why SlickGrid move focus to focusSink element .
But I think it doesn't need to work when any element don't have focus( = only works when some element have focus).

I wrote jsFiddle sample (confirmed by Chrome & IE8-10).
http://jsfiddle.net/icoxfog417/PC8Mc/