Patch from Chris Shoemaker to avoid trying to double-add plugins to

[David Hampton]

Log Message:
-----------
Patch from Chris Shoemaker to avoid trying to double-add plugins to
plugin manager by delaying signal connection until the plugins are
already managed by the plugin manager.

Tags:
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gnucash-gnome2-dev

Modified Files:
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    gnucash:
        ChangeLog
    gnucash/src/gnome-utils:
        gnc-main-window.c