[cwvreg] cleanup the event manager when reloading modules Closes #3848995 The event manager callbacks are not cleaned during reloading. So the callback defined in the reloaded module appears twice (old and new version). This may cause problem when the old version is called.

import ConfigParser
try:
    undo_actions = config.cfgfile_parser.get('MAIN', 'undo-support', False)
except ConfigParser.NoOptionError:
    pass # this conf. file was probably already migrated
else:
    config.global_set_option('undo-enabled', bool(undo_actions))