Fix checkbox font clipping (kallisti5#126)

* Use const in more places

* Revert "Use const in more places"

This reverts commit 125b4aa.

* Fixed checkbox widgets would clip the font height

At least in some cases (with a font size of 15 that I tested), the font height was clipped due to the height of the checkbox being smaller.
Adding 2 pixels of padding in the height seems to fix that.

src/widgets/checkbox.cpp

@@ -192,7 +192,8 @@ namespace gcn
     {
         const int height = getFont()->getHeight();
 
-        setHeight(height);
+        // Add two pixels to avoid font clipping
+        setHeight(height + 2);
         setWidth(getFont()->getWidth(mCaption) + height + height / 2);
     }