files-view: Only set discrete scrolling when ctrl is held
The scroll controller on the scrolled window in files view is set to discrete in order to accomodate for when CTRL+scroll (zoom) is performed. Without setting the SCROLL_DISCRETE flag, it would be very difficult to make minor zoom adjustments. This however, causes problems with scrolls that have delta's <1 (especially when they are much much less 1). So instead let's set the discrete flag only when CTRL is held and remove it otherwise.
Fixes: #2511 (closed)