This interface is to be implemented by all plugins that want their configuration
to be saved and restored to XML.
Each plugin is responsible to save any item that it created in the MasterTree,
in the window manager, in plot pages etc etc.
The reloading of the configuration is done in steps to ensure that all the
object dependencies are properly handled. The first step of the reloading process
is to ask all the plugins to open the data structures on which all the other
objects (like plots, pages, editors) depend on. Each plugin is called for the
initialization phase and should tell the XMLIOManager at which restore level
it should be called back.
int restore(int level,
level is the level at which the restore should be performed.
0 - the initial level. At this level all the data should
be restored. This level MUST be present!!!
more then 100 - final state restoring: finishing touches on the tree,
order of the panels on the plot area and console area.