Improve FileChooser a11y

My ultimate goal is to be able to drive GtkFileChooserDialog through a11y to open a file when we know its path, ie. using the Location text field which is not visible by default.

This MR gives GtkFileChooserWidget enough a11y to be controlled for this, and helps to find the GtkFileChooserWidget inside the GtkFileChooserDialog. In use for scripts/uitest in https://gitlab.com/omaha-bgg/omaha/-/tree/toolkit/gtk3

The add a couple of accessible names in .ui file commit is not strictly necessary to restore a11y there, but it is useful to dig into problems as described in https://ydirson.gitlab.io/the-floss-cook/a11y/2020/10/28/pyatspi-gtkfilebrowser.html