Categories: Port away category parsing from QDom

Aleix Pol Gonzalezrequested to merge
work/apol/categories-qxmlstreamreader into master

These classes are both slower and deprecated in Qt 6, so we better port away from them.

Test Plan: We need to make sure categories stay the same and apps keep being properly filtered when navigating categories. I couldn't find any regressions.

Edited by Aleix Pol Gonzalez

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