Merge branch 'server-search'

Akonadi server now supports searching via 3rd party search plugins (for
example Baloo plugin), which means we can retrieve results very quickly,
while not having to depend on 3rd party code, like we did with Nepomuk.

More importantly, we now support so called server search (or agent search).
Search results are not retrieved only from the search plugins, be the server
will also query agents and resources that implement AgentSearchInterface
to retrieve search results from their storage search service (for example
IMAP SEARCH extension). This makes it possible to retrieve search results even
for items that don't have full payload cached in Akonadi and therefore cannot
be indexed by Baloo or Nepomuk.

Big thanks to Christian Mollekopf for his help with implementing this.

Conflicts:
server/src/handler/search.cpp