Factor out common code for handling menus, popup menus, and the toolbar

[David Hampton]

Log Message:
-----------
Factor out common code for handling menus, popup menus, and the
toolbar and move the code to the shared base class.  Rework the
private data structure in gnc-plugin-page.c.

Tags:
----
gnucash-gnome2-dev

Modified Files:
--------------
    gnucash:
        ChangeLog
    gnucash/src/business/business-gnome:
        gnc-plugin-page-invoice.c
    gnucash/src/business/business-gnome/ui:
        gnc-plugin-page-invoice-ui.xml
    gnucash/src/gnome:
        dialog-scheduledxaction.c
        dialog-sxsincelast.c
        gnc-plugin-page-account-tree.c
        gnc-plugin-page-register.c
        gnc-plugin-page-register.h
    gnucash/src/gnome/ui:
        gnc-plugin-page-account-tree-ui.xml
        gnc-plugin-page-register-ui.xml
    gnucash/src/gnome-utils:
        gnc-main-window.c
        gnc-main-window.h
        gnc-plugin-page.c
        gnc-plugin-page.h
    gnucash/src/gnome-utils/ui:
        gnc-main-window-ui.xml
    gnucash/src/report/report-gnome:
        gnc-plugin-page-report-ui.xml
        gnc-plugin-page-report.c