* src/workbook-control-gui.c (wbcg_finalize,
wbcg_close_if_user_permits):  Move call of
x_store_clipboard_if_needed from finalize to
close_if_user_permits. This works around some lifecycle issues.

* src/gui-clipboard.[ch] (x_store_clipboard_if_needed):
Make it take a workbook, not a wbcg. Should now be explicitly
called before workbook object is destroyed.