fix #286144 : no or wrong style assigned to texts in parts of 2.x scores

[AntonioBL]

Resolves: https://musescore.org/en/node/286144
(even if that bug is a mixture of this and https://musescore.org/en/node/285434 )

When reading user styles from 2.x scores, the list of used styles was never reset before reading new scores (such as the scores of the different parts, if present). Therefore user-defined styles could be lost for the parts because of "addUserTextStyle: too many user defined textstyles".

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