Add a button to the todo list plugin to remove objects that don't exist anymore

[Polyglot]

While validating, I add objects for schools to the todo list after performing a search. Then I find that I'm merging 30% or so in some cases, which means that those are not found anymore, when it's their turn to be processed.
There are 2 solutions for this

1. remove them from the todo list automatically if they stop existing in the data layer
2. add a button that starts an action to go trough the list and remove the objects that aren't present anymore in the data layer (they could be purged too)

Jo