make the "ignore acceleration" controlled by optional boolean flag

The flag defaults to false, so that strict backwards compatibility
is maintained. To resolve the issues mentioned in bug #21335, the
macros such as at

https://github.com/wesnoth/wesnoth/blob/5b72fe99a50bbdece9b540af74c652ab3c7d5a24/data/core/macros/interface-utils.cfg#L36

should be updated to include "ignore_acceleration=false" when they
use [delay] tags.