I've worked with a lot of different versions of FileMaker going back to FMP 2.5 and have never seen such a list as a built in feature. Might it have been a plug in?
I've seen a lot of "button templates" over the years where you can copy and paste them, but not a list of them built in.
I think it was on Mac, I watched a vedio before where I saw this function. May be it's a plug in.
This is a FP5 database, would require conversion to FP7, then FP12
Google "buttons filemaker"
Back when I had a Mac in the pre OS X days, there was a widget in the apple menu that set up a "scrap book" of graphic items from/to which you could copy/paste items. Perhaps some such utility was what you remembered?
Extract Graphics from Application
http://www.pragmaticcode.com/crunch/ 15 day trial