Update and improve "Hello world" example in the documentation.

The example was corrupted by transition to Doxygen (the menu item labels got
eaten), fix it to actually work.

Also use this opportunity to improve and modernize it.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@67006 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775