bug#30786: Save text properties in desktop

[Juri Linkov]

>> Why does desktop.el not save text properties?
>> Maybe for some historical reasons?
>> This is required for bug#22479.
>> I can't find a problem while trying this patch:
>
>> -     (let ((copy (copy-sequence value)))
>> -       (set-text-properties 0 (length copy) nil copy)
>> -       ;; Get rid of text properties because we cannot read them.
>
> I guess this comment is relevant, you can easily have non-readable
> property values, e.g.: (put-text-property 1 2 'foo (point-marker)).

If the only problem is with non-readable property values then we could
check for such values and not to write them to the desktop file.

> Maybe solving Bug#24982 would help?

This would help on reading the desktop, but maybe better not to save
non-readable values in the first place.