Prevent adding same folder and file to workspace #775 #1588 #1217

This is a way to stop appearing bugs with duplicate folders and files in workspace.

I'm not doing anything with existing duplicates in workspace. So user would have to clear workspace and add back all the things.
You can just do (:files @this) instead of (:files (-> @this)). Also, let's give the user an error message when they do this. This will require changing when-not to if and for the false case to add `(notifos/set-msg! "This file/window is already in your workspace." {:class "error"})`