Implement support for server-side search in Akonadi

Instead of using Nepomuk and SPARQL queries, we now use SearchQuery and SearchTerm objects
to construct queries (internally represented as a JSON query language based on Baloo's
query language.

We now also support server-search, so agents that implement AgentSearchInterface can search
their storage service (like IMAP SEARCH for example) and return results back to Akonadi. This
allows us to search even items that are not indexed by a local indexing service, like Nepomuk
or Baloo.

Big thanks to Christian Mollekopf for his help

Squashed commit from akonadi/server-search branch.