This signal is emitted when the user "activates" a file in the file chooser.
This can happen by double-clicking on a file in the file list, or by pressing `Enter`.
Normally you do not need to connect to this signal. It is used internally by FileChooserDialog to know when to activate the default button in the dialog.
See also: get_filename, get_filenames, get_uri, get_uris.