searchDisplay: Split renderResults()

searchDisplay: Split renderResults()

renderResults() updates the results set, determines the number of
results to display, retrieves the corresponding result metas and
adds a new results actor for each meta.
Splitting the function in those parts allows to move the retrieval
of the result metas into SearchResults, which is where we ensure
flicker-free rendering and control the selection - we want to keep
both features for asynchronous result metas which we are about to
introduce.

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