Add possibility to use images as background or handle of slidevalue

* Add tag <resource background="x" handle="y" handlehover="z" />
  to the slidervalue object.
* All images are optional, e.g. you can use just handle or just
  background image or no images whatsoever

Change-Id: Ie0fee92965c6e883c4e532732756008dcc9e385e
Signed-off-by: Vojtech Bocek <vbocek@gmail.com>
diff --git a/gui/objects.hpp b/gui/objects.hpp
index 486e2e7..cf20d7e 100644
--- a/gui/objects.hpp
+++ b/gui/objects.hpp
@@ -999,7 +999,11 @@
 	int mFontHeight;
 	GUIAction *mAction;
 	bool mChangeOnDrag;
-	int lineW;
+	int mLineW;
+	bool mDragging;
+	Resource *mBackgroundImage;
+	Resource *mHandleImage;
+	Resource *mHandleHoverImage;
 };
 
 class MouseCursor : public RenderObject