usability: please forward more cursors

[onlyjob]

0.8.2: there are many applications where in-window frames can be resized by dragging border. In none of those apps I see mouse cursor turning its shape when hovering over border.

It would be nice if mouse cursor would change its shape within Xpra session so users could see where they can start dragging rather than guessing the sensitive area without visual feedback.

Thanks.

[Antoine Martin]

Dupe, see #256