Geary.App.FillWindowOperation: detect stale FillWindowOperations
When a user types in the search box, there may still be oustanding FillWindowOperations queued on previous instances of SearchFolder from previous keystrokes. This can result in a FillWindowOperation with a stale value of ConversationMonitor.window_lowest from a previous search executing on the current one. Detect this and queue a new FillWindowOperation if needed.
Fixes: #838 (closed)