[Doxygen-develop] Add option to layout diagrams vertically

[Pietro M. <pie...@gm...>]

Inheritance diagrams are drawn horizontally, and can extend way beyond the
width of the page, forcing the reader to scroll to the right.

It would be useful to add the option to lay out them vertically in case
they are too wide, e.g. if a base class has more than five derived classes,
or if the total width of the derived classes is above a predefined value.

This would provide the classic horizontal layout most of the times, and
just use a vertical one when necessary.