BColumnListView: Enable invalidating rows, fix SetField()

* Seems like there was no easy way to simply invalidate
   a given BRow. Introduced BColumnListView::InvalidateRow().
 * BRow::SetField() tried to invalidate the row, but invalidated
   the listview instead of the BOutlineView responsible for
   drawing the list contents. Use the new InvaalidateRow().

headers/private/interface/ColumnListView.h

@@ -135,6 +135,8 @@ public:
 			bool 				IsExpanded() const;
 			bool				IsSelected() const;
 
+			void				Invalidate();
+
 private:
 	// Blows up into the debugger if the validation fails.
 			void				ValidateFields() const;
@@ -332,6 +334,8 @@ public:
 									parentRow1 = NULL, BRow* parentRow2 = NULL);
 			void				Clear();
 
+			void				InvalidateRow(BRow* row);
+
 	// Appearance (DEPRECATED)
 			void				GetFont(BFont* font) const
 									{ BView::GetFont(font); }