icons, libgimpwidgets: now with proper gimp-attach + gimp-detach icons.

"gimp-detach" does not just use "gtk-convert" anymore and has its own
design. As for "gimp-attach", this is not used anywhere (yet), but it
could be soon as reverse action of gimp-detach. For instance, this icon
can be used for bug 791859 when we implement re-attaching overlay
dialogs.