This is a small performance improvement.

The callgrind session made by Thomas Zander shows that these two functions are needing quite a lot of time.

Using a QHash<QString, KoParagraphStyle*> to store the name and the paragraphStyle in the style manager is a faster way to do that.

On my computer, the performance improvement when loading the ODF 1.2 draft 3 spec is about 12% (20 seconds).