I am trying to set Super+T to start an application, but that shortcut is not recognized. I guess it is already in use somehow, but I don't see it in the list of shortcuts and when I press it nothing happens. Is there a way to list all shortcuts currently in use?
You should be able to search your keyboard shortcuts by pressing the magnifying glass button while in the settings menu. Not quite a list, but at least you can see what is in use.
However I think this won't solve your problem. Not all key combinations are valid for custom shortcuts, and I don't think '+t' is allowed. A user posted some limits here, but I haven't seen an exhaustive list of what is or isn't permitted.