Remove preview widget from GtkFileChooser

Remove preview widget from GtkFileChooser

The preview widget harks from a platform before time, when we didn't
have GIO, or a thumbnail specification.

Very few applications use it correctly, if at all; it has an horrid hack
to deal with the ownership of the widget's instance when accessed
through the getter function; it messes up the layout of the widget and
its label is less than useful when it comes to file names longer than a
dozen characters; it's a poor substitute for a proper thumbnail view.