Clean up xml data structures when the dialog is destroyed.

[David Hampton]

Log Message:
-----------
Clean up xml data structures when the dialog is destroyed.  Connect
signals in all add-in preference pages.  Don't show the dialog until
completely built and moved to the remembered position.

Tags:
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gnucash-gnome2-dev

Modified Files:
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    gnucash/src/gnome-utils:
        dialog-preferences.c
        preferences.glade