* src/gui-clipboard.[ch] (x_store_clipboard_if_needed): New
function. Hand clipboard off to clipboard manager if this is the
last remaining wbcg.

* src/gui-clipboard.c (table_cellregion_write): Replace wbc
parameter with a command context.
(image_write, graph_write): Remove sheet parameter.
(x_clipboard_get_cb): We're no longer passed a wbcg. Instead,
create a temporary command context. Test output from
gnm_cellregion_to_xml before using. Use workbook_date_conv from
workbook of clipboard's origin_sheet.
(x_clipboard_clear_cb): Simplify.
(x_claim_clipboard): Register x_clipboard_clear_cb only for
CLIPBOARD, not for PRIMARY. Call gtk_clipboard_set_can_store,
indicating that we can hand clipboard off to clipboard manager
when exiting.

* src/workbook-control-gui.c (wbcg_finalize): Call
x_store_clipboard_if_needed.