Draft: Issue #6500: Notify GimpImage when all layers have been deselected

This attempts to resolve issue #6500.

You can deselect layers in the layer dockable by holding Ctrl when clicking. While you can deselect all layers this way, GimpImage's select layers list is not updated unless at least one layer is still selected. This can result in unexpected behaviors as noted in the report.

This patch moves the "at least one selected layer" check in gimp_item_tree_view_select_items () so it still calls set_selected_items () with a NULL value if there are no selected layers.