Emphasise how to delete message custom property

[lukasz-pyrzyk]

The repair and resubmit message window allows us to edit the message. Unfortunately, it's quite difficult to find a way to remove a property. It looks like it works only by clicking Delete on the keyboard, which is not the best experience. I found it only by guessing.

I would be great to have a delete action with the mouse (maybe after right-click) or a label somewhere below the custom properties saying something like

Press DELETE key to remove the property.