overviewControls: Fix bad expression causing incorrect thumbnails allocation

`a + b ? c : d` is parsed as `(a + b) ? c : d`, not the more intuitive
`a + (b ? c : d)`.

This was causing a bad slide animation and Clutter warnings when coming
out of the overview.
remoteSearch: Do filtering here of providers here

We already do ordering at load time, so why not filtering?
search: Make the internal search interface callback-based

Long ago, the search system worked in a synchronous manner: providers
were given a query, and results were collected in a single array of
[provider, results] pairs, and then the search display was updated
from that.

We introduced an asynchronous search system when we wanted to potentially
add a Zeitgeist search provider to the Shell in 3.2. For a while, search
providers were either async or sync, which worked by storing a dummy array
in the results, and adding a method for search providers to add results
later.

Later, we removed the search system entirely and ported the remaining
search providers to simply use the API to modify the empty array, but the
remains of the synchronous search system with its silly array still
lingered.

Finally, it's time to modernize. Promises^WCallbacks are the future.

Port the one remaining in-shell search engine (app search) to the new
callback based system, and simplify the remote search system in the
process.