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 heavys 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.

This patch reduces the Remove all content step of the benchmark from ~25s to ~4s.