(dw_locktimer) plugins may reuse to add fields and callbacks

Plugins may want to add further fields to be present when saving drafts.
Plugins may want to execute some js functionality that should be timed to
the saved draft/refreshed lock.

If a plugin does another init() to attach the dw_locktimer to its own editor,
then the default callback would be added a second time, causing unexpected
and undesired behavior.

This includes the changes from the following commits:
6ef45cc1
6ca947f3
0fff419c
87bed8b6