The down_folder signal is a keybinding signal which gets emitted when the user asks for it.
This is used to make the file chooser go to a child of the current folder in the file hierarchy. The subfolder that will be used is displayed in the path bar widget of the file chooser. For example, if the path bar is showing "/foo/bar/baz", with bar currently displayed, then this will cause the file chooser to switch to the "baz" subfolder.
The default binding for this signal is `Alt + Down`.