Adds classes that allow building arbitrary query expressions and require
an instance to be provided to Geary.SearchQuery, to be set as a
property.
This enables query expressions to be parsed by clients instead of the
engine, in which ever whay they choose.
Engine mocks don't need to be in the `Geary` namespace, and including
them there makes it difficult to use them in client tests, so put them
all in their own name-space and corresponding directory.