scrollbar, options and widgets changes

* complete scrollbar skin
* implement scrollbar functionality (scrolling with mouse)
* fix onMouseClick issues
* add tabs in options (graphics and general tab)
* add new option for limiting frame rate using scrollbar
* add new widget property "clipping" that will be used on scrollable areas

modules/core_lib/widgets/uiscrollarea.lua

@@ -0,0 +1,15 @@
+UIScrollArea = extends(UIWidget)
+
+function UIScrollArea.create()
+  local scrollarea = UIScrollArea.internalCreate()
+  scrollarea:setClipping(true)
+  return scrollarea
+end
+
+function UIScrollArea:onStyleApply(styleName, styleNode)
+  for name,value in pairs(styleNode) do
+    if name == 'horizontal-scrollbar' then
+      
+    end
+  end
+end