fdo#76260 - switch O(N^2) lookup in SwStyleSheetIterator to O(N)

The SwStyleSheetIterator is called a lot on import of DOCX;
potentially another N times - so this change saves 15%+ of load time,
81bn cycles of 457bn to startup and load the document.

Change-Id: I70ef0f1ebd3f4e05519be68c8a67f65b00f54719