bug#29279: Sharing the margins

[martin rudalics]

> If the latter doesn't have to run window-configuration-change-hook,
> maybe the margin changes don't have to either, and "other hooks" will
> suffice as well?

I intend to drop most calls of 'window-configuration-change-hook' and
replace them by 'window-size-change-functions' if all they do is to
change the sizes of windows only.  Maybe we should then change the
semantics of the latter: Call it when just the window body size changes
too.  Provide a 'window-body-pixel-width-before-size-change' function
which could be used in connection with 'window-body-height'.

martin