* src/wbcg-actions.c (cb_insert_image): is_save parameter to
gui_image_file_select no longer needed.

* src/sheet-object-image.c (soi_gdk_pixbuf_save): New. Callback
which writes via gsf.
(soi_get_image_fmt): New. Get a format descriptor for the original
data in a SheetObjectImage.
(soi_free_image_fmt): New. Free a format descriptor.
(soi_cb_save_as): New. Choose file name and format and save
image.
(sheet_object_image_populate_menu): New. Put a "save as" entry in
the object's context menu.

* src/sheet-object-graph.c (soi_cb_save_as): Choose image format
from a list.

* src/gui-file.h (GnmImageFormat): Struct which describes an
(image) file format.

* src/gui-file.c (gui_image_file_select): Split the parts common
to save and open into new function gui_image_chooser_new. Drop
is_save parameter to _select.
(gui_get_image_save_info): The parts of _select which have to do
with saving go here. Add a format chooser.

* graph/gog-style.c (cb_image_file_select): is_save parameter to
gui_image_file_select no longer needed.