search: Make asynchronous providers more explicit

Currently, asynchronous search providers are expected to call
startAsync() in getInitialResultSet()/getSubsearchResultSet(),
which will trigger async mode until the search is canceled or
updated. Switching between synchronous and asynchronous mode like
this makes asynchronous search an implementation detail, but being
transparent to the searchDisplay means that certain optimizations
don't work as expected. Namely, updating asynchronous search results
causes flickering, and the automatic selection never focuses
asynchronous results.
So change the API to require providers being either synchronous (with
the current getInitialResultSet()/getSubsearchResultSet() methods)
or asynchronous (with asynchronous variants), and handle asynchronous
providers explicitly in searchDisplay.

https://bugzilla.gnome.org/show_bug.cgi?id=663125