* src/common/plugin.c
fix unloading of all plugins and unload
plugins in reverse order
* src/gtk/pluginwindow.c
implement function of load and unload button
* src/gtk/prefswindow.c
* src/gtk/prefswindow.h
rename register function and add new unregister
function
* src/plugins/spamassassin/spamassassin_gtk.c
unregister prefs page and update for changes in
src/gtk/prefswindow.c