bug#25851: 25.2; GTK warning when starting Emacs when desktop file has more than one frame

[martin rudalics]

> If frame positions are not restored in general (again, modulo
> any necessary exceptions that might exist) that would suggest
> that there is little point in saving and restoring frames.
>
> Size and position are among the most important attributes of a
> frame, and if a user bothers to use multiple frames then it is
> almost certainly the case that their sizes and positions are
> important.

Jan who was responsible for the implementation of this in the past
advocated the use of Extended Window Manager Hints where you can read
(in the section about _NET_WM_FULL_PLACEMENT) that

  Clients, when they detect that this hint is supported, SHOULD NOT
  abuse or often even use PPosition and USPosition hints for requesting
  placement.

  Rationale: Window managers can often perform better placement (that
  may be even configurable) for windows than the application. However at
  the time of writing this it is problematic for Window managers to
  decide when to use them because many applications abuse positioning
  flags and/or provide unnecessary default positions.

So it's up to us to either go this direction or do our own placement.  I
neither use desktop saving nor explicit window positioning so I have no
preference.  But as this bug demonstrates, it seems that we may have to
choose one or the other eventually.  Otherwise, we will sooner or later
end up in situations that are more troublesome than the present one.

martin