Scroll full pages with mouse wheel

When using the scroll wheel over a page, holding Shift will cause full
pages to be scrolled instead of scroll steps. The same effect can be
achieved by scrolling over the vertical scrollbar.

doc/user_help.in.html

@@ -188,6 +188,7 @@ <h3 id="scrolling">Scrolling</h3>
   <li>Using the <em>mouse</em> or <em>pointing device</em>:
     <ul>
       <li>Rotate the mouse wheel to move one <em>step</em> in that direction.
+          Hold Shift to move one <em>page</em> instead.
       <li>Keep the middle button (or mouse wheel button) pressed while
         dragging to scroll the page precisely.
     </ul>
@@ -202,6 +203,8 @@ <h3 id="scrolling">Scrolling</h3>
         specific position of the page.
       <li>Click the scrollbar arrows to move the view one <em>step</em> in
           that direction.
+      <li>Rotate the mouse wheel over the vertical scrollbar to move one
+          <em>page</em> in that direction.
     </ul>
   </li>
 </ul>

dw/fltkviewport.cc

@@ -348,6 +348,16 @@ int FltkViewport::handle (int event)
       break;
 
    case FL_MOUSEWHEEL:
+      if ((vscrollbar->visible() && Fl::event_inside(vscrollbar)) ||
+            Fl::event_shift()) {
+         if (Fl::event_dy() > 0) {
+            scroll(core::SCREEN_DOWN_CMD);
+            return 1;
+         } else if (Fl::event_dy() < 0) {
+            scroll(core::SCREEN_UP_CMD);
+            return 1;
+         }
+      }
       return (Fl::event_dx() ? hscrollbar : vscrollbar)->handle(event);
       break;