Add multiplication sign for time signatures

[rettinghaus]

This PR makes the multiplication sign for time signatures available:

Upper and lower case X may be used as well as the multiplication character (×) ×.

[Jojo-Schmitz]

Can't that get extended to also allow -, and timeSigMinus (and then change timeSigPlusSmall to timeSigPlus?
Maybe also = and /, timeSigEquals and timeSigFractionalSlash?

[Jojo-Schmitz]

Any special reason why not using the characters themselves rather than their ASCII code?
As far as I can tell Char(88) is the same as Char('X'), except the latter is more readable (and obsolets the comment)
Or Char(u'×'), etc.

[rettinghaus]

Can't that get extended to also allow -, and timeSigMinus? Maybe also = and /, timeSigEquals and timeSigFractionalSlash?

I think those are for compound meters like "3/4= 6/8" and shouldn't turn up in this context.

[Jojo-Schmitz]

I believe similar additions are needed here:

MuseScore/src/palette/view/widgets/timesignaturepropertiesdialog.cpp

Line 83 in a0e3c29

QRegularExpression regex("[0-9+CO()\\x00A2\\x00D8\\x00BD\\x00BC]*");

to be able to actually enter those new signs

[rettinghaus]

@Jojo-Schmitz interestingly the time signature dialog you open from the palettes is not limited by a similar expression.

[Jojo-Schmitz]

But the timesig created with it is! It lacks those symbols.

See also my backport to Mu3, Jojo-Schmitz#765