Right now titles aren't checked when merging two sessions. This requires changes in both SessionHandler.canMerge() and SessionHandler.canMergeAll()

Restrictions on sessions is softening up with #545, which I think removes the desire to have this check.

What behavior should occur instead?

The two can merge methods should check for any duplicate titles in the afflicted sessions, and if it does find any collisions, the whole method should return false.