Subfiltering step: handle text units that include overlapping runs

[Kuro Kurosaka]

This issue is for the Subfiltering step that will be added when pull request #300 is merged. Creation of this issue is one of the the code review comments by @Denis Konovalyenko .

When text unit, which already includes placeholders, is fed to the Subfiltering step, that could result in overlapping runs (see issue #791). This is very difficult to handle. The first version of the Subfiltering step in PR #300 does not handle the situation. Instead, if the applying the subfilter would result in overlapping runs, it gives up subfiltering and just returns the original text unit. We should develop a complete solution that can handle the overlapping runs.