LIB: make interaction with FileOperation more safe

We now don't safe an AnchoredFile anymore, but a Path and
construct the AnchoredFile just before the operation is carried out.

This means the copy and move buffers cannot contain stale references
to files that don't exist anymore.

src/HSFM/GUI/Gtk/Data.hs

@@ -101,7 +101,7 @@ data MyView = MkMyView {
   , rawModel        :: TVar (ListStore Item)
   , sortedModel     :: TVar (TypedTreeModelSort Item)
   , filteredModel   :: TVar (TypedTreeModelFilter Item)
-  , operationBuffer :: TVar (FileOperation FileInfo)
+  , operationBuffer :: TVar FileOperation
   , inotify         :: MVar INotify
 }