Add GimpCellRendererButton and use it to add a "Save" icon to each row
of dirty images. Click invokes the "edit-save" action, shift-click
invokes "edit-save-as". Also add a tooltip for the icon button.

Involves minor changes to GimpContainerTreeView to allow
GimpCellRendererButton to be added, and to allow external
"query-tooltip" handlers to run.