UserActionsMenu: Use an icon for the "Move to Desktop" -> "New Desktop" action

Taken from a pager applet, where all actions have icons. Worth porting to task manager as well.

There is one concern that is shifts all the labels to the right because menu allocates space for both checkboxes (radiobuttons) and icons, even when they do not overlap. I believe this is mostly a styling issue, not a conceptual one, and it should be reconsidered.