Add hooks to run before and after the popup appear

[lthms]

This is a simple proposal to have a way to run custom code when the popup is created and when it is deleted.

Here is how I use it:

(use-package eldoc-box
  :hook
  (eldoc-mode . eldoc-box-hover-at-point-mode)
  (flycheck-popup-tip-show . (lambda ()
                               (when eldoc-box-hover-at-point-mode
                                 (eldoc-box-hover-at-point-mode -1))))
  (flycheck-popup-tip-delete . (lambda ()
                                 (unless eldoc-box-hover-at-point-mode
                                   (eldoc-box-hover-at-point-mode)))))

Thanks to that, there is no conflict anymore between eldoc-box and flycheck-popup-tip.

Feel free to merge or close this PR, as you see fit.

Thanks for flycheck-popup-tip.

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