Bugfix: don’t change tabbed/stacked cons to splitv/splith layout (Thanks Merovius)

Bugfix: don’t change tabbed/stacked cons to splitv/splith layout (Thanks Merovius)

To automagically do the right thing when rotating monitors with regards
to splith/splitv layout (depending on width/height of the monitor), we
change the orientation of existing workspaces and the first child.

If that first child happens to be a stacked/tabbed con, we cannot change
the layout unconditionally (previously, the orientation was not in the
layout, so we never noticed this problem).

fixes #768