Move mousedown handler completely out of event handlers.

This fixes #409. The way it was it would add and remove the mousedown
handler multiple times (for each plugin that uses popovers). It seems a race
condition then ensured that it wasn't added at all. With this change, it is
added once when the module is loaded.
Merge pull request #40 from wysiwhat/oerpub-ticket409

Move mousedown handler completely out of event handlers.