Cursors: Add new Eyedropper cursor

This can be used immediately in PixelPaint (separate commit), but
I am adding this as a system-wide cursor since it may also be useful
for other applications that want to use it.

Userland/Libraries/LibGfx/StandardCursor.h

@@ -26,6 +26,7 @@ enum class StandardCursor {
     Move,
     Wait,
     Disallowed,
+    Eyedropper,
     __Count,
 };