Do you mean something like in the attached screenshot? This is from Geany 0.3 where we had a combo box in the toolbar containing all the symbols which are also in the symbol list sidebar are shown. I don't remember the details but it caused to many troubles in the toolbar so we or I removed it. But since the whole corresponding has changed a lot since this time (this was more than 3 years ago, IIRC), maybe it's worth to give it another try. Maybe not in the toolbar but instead somewhere else. Ideas?
Three years ago I was only programming in mcedit ;-)
I don't think all symbols are such a good idea. The best would be, if user could define, what he want to have displayed (classes, methods, globals, etc.) and display only those elements.
I have no idea, where else it could be put. Toolbar seems a natural place.