FileChooserWidget: Avoid crash freeing static str

FileChooserWidget: Avoid crash freeing static str

Having a FileChooserDialog in location-entry mode then pressing
<primary>f to move to search mode would crash with an invalid free().

In that case, FileChooserWidget.get_subtitle() returned a static string
straight from gettext. This crashed when the GBinding from :subtitle to
FileChooserDialog’s HeaderBar:subtitle shortly tried to free the string.

Fix by duplicating the string before returning it, like all other paths.

https://bugzilla.gnome.org/show_bug.cgi?id=791004