mode-line menu for minor modes

[Dan Nicolaescu]

The new feature to pop a menu when clicking on the mode-line :lighter for
minor modes is great.

But for minor modes that don't provide a menu, just an error message is
displayed: "No menu for minor mode `BLAH'".  This is not very helpful
for the user, it would be nicer to pop up a menu that has (at least) 2
entries: - one to turn off the minor mode
         - one to show the help for that minor mode.

Yes, these are available elsewhere, but it's better to provide something
useful and consistent for mouse-1 instead of just complaining that a
menu is not available.