Padre will have a lot of plug-ins. First plug-in manager was not taking
this into account, and the first plug-in manager window was too small &
too crowded to show them all properly.
This revamped plug-in manager is now using a list_two control, and thus can
show lots of plug-ins in an effective manner.
Upon selection, the right pane will be updated with the plug-in name &
plug-in documentation. Two buttons will allow to de/activate the plug-in
(or see plug-in error message) and set plug-in preferences.