Adds a folder to be displayed with the shortcut folders in a file chooser.
Note that shortcut folders do not get saved, as they are provided by the application. For example, you can use this to add a “/usr/share/mydrawprogram/Clipart” folder to the volume list.
filename of the folder to add
true if the folder could be added successfully, false otherwise. In the latter case, the throws will be set as appropriate.