Small but handy GuiScrollCtrl enhancement

what:
1. If you press SHIFT, then the scroll will do horizontal scrolling instead of vertical
2. If there is nothing to scroll vertically, it will try to scroll horizontal automatically

Changes:
open:
guiScrollCtrl.cc

locate:
bool GuiScrollCtrl::onMouseWheelUp(const GuiEvent &event)
change:

scrollByRegion((event.modifier & SI_CTRL) ? UpPage : UpArrow);

to this:

scrollByRegion(
      (event.modifier & SI_SHIFT || !mVBarEnabled) ? 
      ( (event.modifier & SI_CTRL) ? LeftPage : LeftArrow) :
      ( (event.modifier & SI_CTRL) ? UpPage : UpArrow )
      );

locate:
bool GuiScrollCtrl::onMouseWheelDown(const GuiEvent &event)
change:

scrollByRegion((event.modifier & SI_CTRL) ? DownPage : DownArrow);

to this:

scrollByRegion(
      (event.modifier & SI_SHIFT || !mVBarEnabled) ? 
      ( (event.modifier & SI_CTRL) ? RightPage : RightArrow) :
      ( (event.modifier & SI_CTRL) ? DownPage : DownArrow)
      );

Save. Rebuild. Enjoy!