bug#62250: 29.0.60; Allow context menu from text properties to not override everything

[Juri Linkov]

> It would be nice if the context menu defined by a text property had the
> option of adding to instead of overriding the menu specified by
> `context-menu-functions'.
>
> I think this can be done in a backwards-compatible way as follows:
>
> (defun context-menu-map (&optional click)
> ...
> When there is a text property `context-menu-function' at CLICK, it is
> handled as if it were the first entry of `context-menu-functions'.

I already envisioned such request, so the text property is named
`context-menu-function' (singular).  As an improvement we could add
another text property `context-menu-functions' (plural) that will
play more nicely with the variable `context-menu-functions'.
For example, we could append the list from the text property
to the value of the variable before running `run-hook-wrapped'
on the composite list.