Cursors: Add new Magnifying glass cursor

There are a few places in the system where this could be useful,
such as PixelPaint and the MandelBrot demo. It seems general enough
that it is probably useful to have it as a system-wide cursor rather
than loading it manually each time.

Userland/Services/WindowServer/Cursor.cpp

@@ -111,6 +111,8 @@ RefPtr<Cursor> Cursor::create(Gfx::StandardCursor standard_cursor)
         return WindowManager::the().disallowed_cursor();
     case Gfx::StandardCursor::Eyedropper:
         return WindowManager::the().eyedropper_cursor();
+    case Gfx::StandardCursor::Zoom:
+        return WindowManager::the().zoom_cursor();
     default:
         VERIFY_NOT_REACHED();
     }