Refactored the location of the progress_callback so that the UI can
specify it and ensure it is used by the automagics process as well as
the plugins themselves.
The progress_callback function has also had an optional description parameter
added, so that it can differentiated in case two scans happen
simultaneously.
This allows NativeSymbolTables to be appended to symbol_spaces and
uses this to let automagic modules (which have no existing symbol table
to rely on) to load natives for reading format data.
Mike Auty