IDML Filter: baseline the least available set of formatting properties on extraction

The idea is similar to issue #887 (closed). It has to improve the translator’s experience with paired tags handling.

The following example should give a better understanding of what can be done.

The very first line should be extracted as Defaults: {FontStyle: Regular, PointSize: 12pt}. Furthermore, the third line has a non-default point size, thus it is currently exposed with surrounding tags: <g>Italic11</g>, however, the targetted result should be without tags: Italic.

For more details please refer to the attached document.

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