When the rename folder menu is opened the text entry is expected to be
focused and selected for a quick editing.
While this is required it doesn't actually happens since PopupMenu by
default gives the key focus to the source actor, that is then free to pass
the key focus to the menu if there's an user interaction.
In this case however, we want the text entry to be focused once we prompt
the menu, so just use the PopupMenu's focusActor property to ensure it will
handle it for us.
Fixes https://gitlab.gnome.org/GNOME/gnome-shell/issues/1604https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/720