Add check for relative tempo markings when tempo text is added from palette

[sejudav20]

Resolves: #15655

Metric modulations taken from the palette contain the default values for tempo, isRelative, and m_relative parameters even though they were initialized when the palette is created. Toggling the "Follow written tempo" or editing the text calls updateTempo which updates these parameters based on the text. I couldn't find why modulations from the palette went to the default parameters but for now, this work around will check the text whether the tempo indicated is relative when the tempo is added from the palette.

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[Jojo-Schmitz]

The failing 3 unit tests chould easily get fixed (i.e. adjusted), but at least the 2 for MusicXML seem to indicate/uncover yet another bug, with 'normal' tempo texts (i.e. non-metric modulations, threse 2 clearly imported wrongly before your change, with a BPM of 320 rather than 80, also cortrected after toggling 'follow text') not having been recognized correctly.
So I'm not 100% convinced your change really fixes the root cause of either issue. Also, as 3 Mu3 has the same isssue, your change doesn't apply to Mu3 code at all, another hint that this might not be the right place?