* src/bonobo-io.c, src/clipboard.c, src/file.c, src/plugin.c,
src/sheet-object.c, src/stf.c, src/workbook.c:
Removed #include "xml-io.h".

* src/dialogs/dialog-plugin-manager.c (gnumeric_dialog_question_yes_no):
Function moved ...

* src/gnumeric-util.[ch]: ... here.

* src/gui-file.c
(gui_file_import): Sorting file importers.
(fill_save_menu): Sorting file savers, function simplified (doesn't use
file_saver_is_default_format).
(can_try_save_to): Function simplified, uses
gnumeric_dialog_question_yes_no from gnumeric-util.h, default answer
for question about overwriting a file is configurable.