Major refinement of Add-ons menu!

* Kill addonsresponselimit, which was a hacky solution to a stupid problem. Instead...
* Allocate and consistently handle memory to store the name of an added file so we can reference it directly.
* Replace the choice between ./ and a custom folder with the full, standard assortment of Default (usehome ? SRB2HOME : SRB2PATH), HOME (SRB2HOME), SRB2 (SRB2PATH) or Custom (cv_addons_folder.string).
* Make these render as the name plus folder, since you can't go UP... from the top level.
* Make the path seperators consistently system-based re PATHSEP. (Quite frankly, I'm surprised it even worked in the first place...)