Don't synchronize browser & WebODF selections unnecessarily

The window selection only needs to be synchronized with WebODF on a
mouse down operation. In all other cases, the selection doesn't need to
be maintained. Removing the selection most of the time significantly
helps performance as there is heavy costs to modifying part of a DOM
tree that is selected.

In order to allow cut/copy to continue to function, an event trap was
necessary that "holds" the selection after mouse-up. This is set as
content editable to allow the cut/copy events to be fired in Firefox and
Safari.
Reset the browser selection on drag end

A drag end will consume the corresponding mouse up operation, meaning
the window selection is not cleared. Cope with this by listening for
the drag event and clearing the selection.