Keep a list of GimpCellRendererViewable around and add API to add
them. When items are removed or the store is cleared, set the
renderers' "viewable" property to NULL so they don't keep refing the
viewable.

This is not really "model" code but needs to be done for all container
views that have viewable cell renderers.

GimpContainerComboBox and GimpContainerEntry lacked that clearing, so
this change might fix some cases where objects (even images) were
removed but still stuck in memory until the model changed again.