improve lacarte.el related stuff

- Introduce a function to create sources with `lacarte-get-overall-menu-item-alist, use its argument

- Use type attribute (command).  C-z will bring up command help.  Because, when a user browses the menu, he will most likely use stuff he doesn't know well.  OTOH it is not really useful to be able to execute multiple menu bar commands in succession

- Advertise the thing more loudly by defining a function `helm-browse-menubar'.  Yes, lacarte.el is not a part of Helm.  But browsing the menubar with helm is a very useful feature  that i think many people would want to use.  We should not hide it in some corner where it's hard to be found.

- I wonder if we should use colors for the key hints in parantheses.