So this patch enables the menuitem.
However, I am not sure this operation is actually supported on this node.
When I used it, all the Trash folders got empties. BUT, their order under the top-level Trash got changed. Also, I then moved some messages to some of the Trashes. But collapsing and uncollapsing the top-level Trash made some of them vanish (even though they contained messages). Changing folder mode to All and then to Unified made them appear again.
Created attachment 8744686
patch
So this patch enables the menuitem.
However, I am not sure this operation is actually supported on this node.
When I used it, all the Trash folders got empties. BUT, their order under the top-level Trash got changed. Also, I then moved some messages to some of the Trashes. But collapsing and uncollapsing the top-level Trash made some of them vanish (even though they contained messages). Changing folder mode to All and then to Unified made them appear again.