Fix icon DnD corner cases

• DnD between two equal-priority monitors wasn't working because setting the icon on the new monitor wouldn't remove it from the config for the old, and then sometimes (if the old monitor was plugged in first), the config for the old monitor would get consulted first.
• DnD of an icon would fail to initiate if the pointer managed to end up over an icon without a motion-notify event getting received first (instead, rubberbanding would start). This could happen when moving an icon to a new position, and then trying to immediately (without moving the pointer first) trying to drag it again, either on the same or a different monitor.