bug#60340: 29.0.60; force-mode-line-update does not update tab line

[Juri Linkov]

>> (defun tab-line-cache-key-default (_tabs)
>>   (list (random 1000)))
>
> Thank you for the pointer! I can do this and it works great:
>
> (set-window-parameter nil 'tab-line-cache nil)
> (force-mode-line-update)

Maybe this call is straightforward enough not to warrant a new special
function to clear the cache.  Anyway, your complex use case prompted
the need to allow more flexibility in tab-line-cache-key-function
that now I improved as well.