Change alt text in Add menu to null (alt="")

When navigating the Add menu, screen reader user hears "Bookmark Bookmark," and because Balsamiq is installed "UI Mockup UI Mockup." When an image is an art element, using alt="" will cause assistive technology to bypass it.

My test method was testing 2.10.3 with a person who is blind, and using "Errors, Features, and Alerts" in the WAVE Toolbar in Firefox.