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/Libraries/LibGfx/StandardCursor.h

@@ -27,6 +27,7 @@ enum class StandardCursor {
     Wait,
     Disallowed,
     Eyedropper,
+    Zoom,
     __Count,
 };