Intelligent stacking of messages in the echo area

[ndame <emacsuser@freemail.hu>]

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If two or more packages use the echo area for informational messages
then often they fight each other, overwriting each other's messages.

For example, in lisp mode eldoc is automatically enabled in my emacs
26.2 If I also turn on a paren mode which shows in the message area
the matching part of a paren outside of the screen then eldoc often
overwrites this message with its own documentation message.

Other systems solve this by stacking messages on top of each other:

https://www.howtogeek.com/wp-content/uploads/2014/11/ximg_54669cfe004ae.jpg.pagespeed.gp+jp+jw+pj+js+rj+rp+rw+ri+cp+md.ic.iDwLFdLQh5.jpg

Maybe the echo area could be smarter and stack messages on top of each
other if they come from diferent sources and they are close to each
other in time.

So in case of the above example, if a message comes from eldoc and
parens at the same time in quick succession then the echo area could
show both of them in two lines.

And if the same source is sending multiple messsages or there is
enough delay between two messages from different sources (e.g. 1-2
seconds) then it would work as today using only a single line.

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