Generalize the the conflict resolution mechanism currently currently implemented by EditPage to work with multiple slots. The 3-way-merge should be applied to each slot separately, using ContentHandler::merge3(). Automatic conflict resolution fails of one of the merges fails.
To satisfy this ticket, it is acceptable to reject edits that change slots other than the main slot if automatic conflict resolution fails. Eventually though, automatic conflict resolution as well as user mediated interactive resolution should be implemented for edits that span multiple slots.