window: Dismiss previous undo toast when undo action has changed

We present a new toast every time the undo action changed which is confusing because you might have several undo toasts appearing over each other. To complicate matters, the previous undo toasts lie (undo'ing would actually undo a different action).

There was a proposal in #2350 to batch undo toasts, but that's problematic because our undo manager is designed to undo exactly 1 action, we do not have an undo stack.

The more straightforward solution seems to be simply dismissing the old undo toasts, when we popup a new undo toast.

Fixes: #2350