* src/sheet-object.[ch]: Add sheet_object_get_target_list.

	* src/sheet-object-image.c (sheet_object_image_get_target_list):
	New. Gets targets object is able render itself as.
	(soi_imageable_init): Register get_target_list virtual.

	* src/sheet-object-graph.c (sheet_object_graph_get_target_list):
	New. Gets targets object is able render itself as.
	(sog_imageable_init): Register get_target_list virtual.

	* src/sheet-control-gui.c (scg_drag_send_image): Use
	go_mime_to_image_format to get format.

	* src/gui-clipboard.c (image_write): Use go_mime_to_image_format
	to get image format.
	(target_list_to_entries): New. Convert a target list to an array
	of GtkTargetEntries.
	(x_claim_clipboard): Use sheet_object_get_target_list to get
	targets for images.

	* src/gnumeric-pane.c (cb_test_so_imageable): Remove
	(target_list_add_list): New. Add targets from "added_targets" to
	"targets".
	(gnm_pane_drag_begin): Use sheet_object_get_target_list to get
	targets for images.