Fix feature overlapping sidebar when dragged or resized.

The overlap is because drag / resize helper is attached to body. While
draggable has an option to attach helper to another element, resizable
doesn't. The only option thus is this hack.

Thanks to Harsh Gupta <gupta.harsh96@gmail.com> for suggesting z-index.

closes #56.

Signed-off-by: Vivek Rai <vivekraiiitkgp@gmail.com>