bug#11680: flyspell should use mouse-3 instead of mouse-2

[Juri Linkov]

>> I think it should be an opt-in behavior, conditional on a user
>> option.  We can later see if it is popular enough to make that the
>> default behavior.
>
> OK.  How about the attached patch?

A small problem: after customizing flyspell-use-mouse-3-for-menu to t,
hovering over a misspelled word displays a misleading tooltip
"mouse-2: correct word at point"
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