add autocmd-event for setloclist/setqflist

[weilbith]

Actual behaviour

There is only an event emitted before and after a command as run that Vim considers to be relevant for the quickfix or location list (e.g. :grep). But the lists can be set by basically everyone, causing no event the user could listen to.

Expected behaviour

An event gets thrown when ever setqflist or setloclist got called.

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Sorry if this was already discussed once. I was not able to find such conversation.
I know there should not be an event for every rare use-case. But in fact this event(s) do already exist, just that they do not apply not for everything. Why emitting an event for specific commands only rather than make it more generic?
To finally make an example where this could be useful would be a status segment that highlights if and how much location lists entry exist. So there is no need to automatically open the location list (if such feature is provided), but the user can have a visual feedback if any asynchronous tool is adding location or quickfix list entries.
Thanks for any input!

[emmanueltouzery]

another example of a use-case is the https://github.com/dhruvasagar/vim-markify plugin, which displays signs for quickfix locations and uses QuickFixCmdPost to listen to quickfix changes, but that even is not triggered for manual setqflist changes.