Allow pages to state whether they should be loaded in the current window

[David Hampton]

Log Message:
-----------
Allow pages to state whether they should be loaded in the current
window or in a new window.  This will soon be controlled by
preferences.

Tags:
----
gnucash-gnome2-dev

Modified Files:
--------------
    gnucash/src/business/business-gnome:
        dialog-invoice.h
        gnc-plugin-page-invoice.c
    gnucash/src/business/business-gnome/schemas:
        apps_gnucash_dialog_business_common.schemas
    gnucash/src/core-utils:
        gnc-gconf-utils.h
    gnucash/src/gnome:
        gnc-plugin-page-register.c
    gnucash/src/gnome/schemas:
        apps_gnucash_general.schemas
    gnucash/src/gnome-utils:
        gnc-main-window.c
        gnc-main-window.h
        gnc-plugin-page.c
        gnc-plugin-page.h
    gnucash/src/register/register-core:
        table-allgui.h
    gnucash/src/report/report-gnome:
        gnc-plugin-page-report.c