* plugins/excel/ms-excel-read.c (excel_read_workbook): Pass filename to
cmd_context_get_password.

* src/dialogs/Makefile.am (base_files): Add dialog-password.c

* src/dialogs/dialog-password.c: Add.
(cb_accept_password): Moved from workbook-control-gui.c.
(dialog_get_password): New function with material taken from
wbcg_get_password in workbook-control-gui.c.

* src/application.c (gnumeric_application_setup_icons): Add
Gnumeric_Protection_Yes_Dialog.

* src/pixmaps/protection_yes_48.png: Add.

* src/pixmaps/Makefile.am:  Add protection_yes_48.png.

* src/workbook-control-gui.c (cb_accept_password): Move to
dialogs/dialog-password.c.
(wbcg_get_password): Most of it moved to dialog_get_password in
dialogs/dialog-password.c.

* src/io-context-gtk.c (icg_get_password): New function which
implements CommandContext::get_password.
(icg_class_init): Connect icg_get_password.

* src/command-context-priv.h (CommandContextClass.get_password):
Ditto.

* src/command-context-stderr.c (ccs_get_password): Ditto.

* src/workbook-control-component.c (wbcc_get_password): Ditto.

* src/io-context.c (ioc_get_password): Ditto.