[MU3] Don't save default settings to pre 3.6 score after 'reset to defaults'

[Jojo-Schmitz]

Resolves: Lengthy discussion in the Developers' chat on Telegram ;-)

Importing a pre-3.6 score into 3.6, and then using Format > Style.. > Reset All Styles to Default, then saving the score does result in all the default settings being written into the score, including the tag <usePre_3_6_defaults>1</usePre_3_6_defaults>.
Closing, opening it again and saving it again (with or without any change results in the score to now (correctly!) not contain any of the default settings anymore, and that <usePre_3_6_defaults>1</usePre_3_6_defaults> is gone too.

With the change from this PR that workaround of a 2nd save is no longer needed.

Helps a lot if you want to open that score in a pre-3.6 version, it then no longer errnously falls back from Leland to Bravura rather then Emmentaler as the Musical font, from Leland Text to nothing for the Musical Text font rather than Emmentaler Text and, worst, from Edwin to "MS Shell Dlg2" (on Windows, on Mac and Linux it'd be some other fallback font) rather than FreeSerif.

What I'm unsure about is the use, need and sense of <metaTag name="mscVersion">3.02</metaTag>, and whether we'd not better get rid of it altogether, or at least remove it again on score write, As we do have <museScore version="3.02"> this seems to be just (very) redundant. But this might be for a different PR...

Counterpart for master is #7508 and got merged meanwhile too

[MarcSabatella]

A clarification - the use Pre 3.6 defaults tag is only written if you don't reset to style, and that is correct. If you do reset to style, then the tag is not written, which is also correct. But style settings are also written to the file unnecessarily, because the scores was never notified it was no longer a 3.5 score. This PR adds the necessary notification to prevent style settings like Edwin from being written unnecessarily.

[MarcSabatella]

What I don't know if that really the exact correct thing to do, or if there is something else that should be updated. But this does seem to basically work.