Clean up search API so that query string parsing and error handling can
be pushed out to the client.
Rename Geary.Account::open_search factory method to ::new_search_query
to better reflect what it actually does.
Update Geary.SearchFolder::search to be async so it can throw errors and
the caller can catch them, require SearchQuery objects instead of plain
text query strings.
Add Geary.SearchQuery::owner property so that the query's account can be
sanity checked.
Update API documentation to reflect this.
c93cfc38b1