Make unicode library a module

So far we have libicu and libunistring implementations, the former used pretty much everywhere and the latter broken for years until this branch. Make both a loadable module (still one implementation has to be picked at build time), so that these unicode libraries are not a direct dependency to users of Tracker for its DBus/remote connections.

This does not make a decision on which to pick as the (still) best candidate is the hairiest dependency (libicu), probably something to do for the next cycle.

Closes: #396 (closed)