Full support for RFC 2047 in subject headers

[BoarGules]

Thunderbird for Android already supports RFC 2047 in the sense that it correctly decodes UTF8 in RFC2047-encoded subject headers, that is, headers containing encoded words of the form =? ... ?=.
But that effort is mostly wasted because it displays any resulting non-ascii glyph as � "unknown". This even affects typographic apostrophes (U+2019, <’>, RIGHT SINGLE QUOTATION MARK) that can be expected from English-language senders.
This is a surprising gap because Thunderbird correctly renders addresses containing such glyphs in international domain name labels, of the form xn--..., as specified by RFC 3490.

[Jon]

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