OpenXML filter can produce duplicate TextUnit IDs in some cases

This bug applies to M28 and may not affect previous versions.

Attached testcase from Andriy Lutskiv. Certain TU IDs (eg "N9C6DE2C0-sub1") are produced more than once.

The structure that produces this is when multiple runs within a single block/paragraph each contain additional nested blocks. BlockTextUnitMapper#processNestedBlocks() creates a new IdGenerator with each call, which resets the "-sub1" suffix.