Frontend for backlog search

We have used a small message object to abstract differences between the telepathy-glib and telepathy-logger message types ever since we started to include backlogs. So far we just used a plain JS object, however we will soon need to represent messages in C code as well, so add a new boxed type that can replace the existing intermediate object. This will promote the existing telepathy-logger dependency from runtime- to build-time dependency, however this is temporary and we will soon get rid of the dependency altogether.