bug#69983: Use category for display-buffer-alist

[martin rudalics]

> 'display-comint-buffer-action' and 'display-tex-shell-buffer-action'
> are out of place in window.el, and they are redundant per se.
>
> What these variables are trying to achieve is already possible
> to do by using a 'category' in 'display-buffer-alist'.

...

> Or we need to keep backward-compatibility?  Then the first step
> to deprecate there variables would be something like this:

Please proceed as carefully as possible, here might be dragons.

martin