fix(i18n): locale-sensitive ordering of folders (#10718)

[winne42]

Closes #10718

Before:

• folders with accented starting letters are moved after the end of the regular alphabet
• lower-case letters are treated as being equal to upper-case --> order is sometimes lower-case < upper-case, sometimes upper-case < lower-case

After:

• folders with accented characters are integrated in regular alphabet
• always lower-case < upper-case

Note that changes to the devices locale may require a restart of Thunderbird to take effect. If this was not wanted, sortForDisplay could be converted from a stored to a computed property.

[github-actions]

Missing report label. Set exactly one of: report: include, report: exclude OR report: highlight.

[wmontwe]

Thank you!

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