Detach search arguments from UI thread

Detach from the UI thread the input arguments used by
the search threads so that the UI thread is able to receive
and process any command sent by the GUI while other threads
keep searching.

With this patch there is no more need to block the UI
thread after a "stop", so it is a more reliable and
robust solution than the previous patch.

No functional change.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>