Convert the invoice dialog over to a GncPluginPage.

[David Hampton]

Log Message:
-----------
Convert the invoice dialog over to a GncPluginPage.  Move the
logic to call the bills due dialog at startup into C code.

Tags:
----
gnucash-gnome2-dev

Modified Files:
--------------
    gnucash/src/business/business-gnome:
        Makefile.am
        business-gnome.scm
        businessmod-gnome.c
        dialog-invoice.c
        dialog-invoice.h
        gnc-plugin-business.c
        gnc-plugin-business.h
        gw-business-gnome-spec.scm
    gnucash/src/business/business-gnome/glade:
        invoice.glade
    gnucash/src/business/business-gnome/ui:
        Makefile.am
        gnc-plugin-business-ui.xml

Added Files:
-----------
    gnucash/src/business/business-gnome:
        gnc-plugin-page-invoice.c
        gnc-plugin-page-invoice.h
    gnucash/src/business/business-gnome/ui:
        gnc-plugin-page-invoice-ui.xml