Avoid hardcoding color in listbox (kallisti5#129)

* Use const in more places

* Revert "Use const in more places"

This reverts commit 125b4aa.

* Don't hardcode listbox inactive color

Instead of hardcoding a specific color for the Inactive one, calculate it based on the Selection color.
This helps when the colors are customized from the defaults.

src/widgets/listbox.cpp

@@ -126,7 +126,7 @@ namespace gcn
             startRow = 0;
         }
 
-        const auto inactive_color = Color(170, 170, 170);
+        const auto inactive_color = getSelectionColor() - 0x303030;
 
         // The y coordinate where we start to draw the text is
         // simply the y coordinate multiplied with the font height.