Ship stock_folder-{copy,move}.png with Thunar itself (bug #6851).

These icons were part of older gnome-icon-theme releases but have now
disappeared. There are no useful equivalents in the icon naming
specification or the GTK+ stock icons (well, there is GTK_STOCK_COPY,
but no appropriate replacement for "stock_folder-move"), so for now it's
best to ship the icons ourselves, I think.