Recognize & handle non-Latin, non-preferred-charset, non-UTF-8 encodings

[eyalroz]

The method BiDiMailUI.Display.resolveDecodingType() currently returns one of:

• "preferred-charset"
• "latin"
• "UTF-8"

and defaults to the latter. Of course, messages may be decoded with other charsets. We should avoid the default, and introduce handling for other charset (= aborting all charset misdetection work).