WindowServer+LibGUI: Support the 4th and 5th mouse buttons

We'll call the "Back" and "Forward" since that's what they are normally
mapped to in some other systems.

Libraries/LibGUI/WindowServerConnection.cpp

@@ -178,6 +178,10 @@ MouseButton to_gmousebutton(u32 button)
         return MouseButton::Right;
     case 4:
         return MouseButton::Middle;
+    case 8:
+        return MouseButton::Back;
+    case 16:
+        return MouseButton::Forward;
     default:
         ASSERT_NOT_REACHED();
         break;