Gnc-Prefs: migrate gnc-plugin gconf machinery

Instead of storing a gconf callback function that will
eventually by attached to the preference when the
plugin is added to a window, simply do this for each
plugin internally in its own add_to_window override
function.
This had already been done in earlier commits for most
plugins. This commit cleans it up for the remaining
plugins and the base plugin code.

git-svn-id: svn+ssh://svn.gnucash.org/repo/gnucash/trunk@23248 57a11ea4-9604-0410-9ed3-97b8803252fd