Currently when you type quickly in the current-file find text edit
it may feel slow because the highlights are updated for each keystroke.

This change increases the minimal delay between highlight updates from
10 ms to 50 ms. That means the delay is still barely noticeable, but
changing the search text stops feeling sluggish.

Change-Id: I41cf2a3282bdbd81afb6f6afb84d52fca16dd184
Reviewed-on: http://codereview.qt-project.org/5876


Reviewed-by: Leandro T. C. Melo <leandro.melo@nokia.com>