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Allow to use UNITY build see https://cmake.org/cmake/help/latest/prop_tgt/UNITY_BUILD.html (Reduce compile time. Need cmake 3.16.0)

Thomas McGuire added many FIXMEs in 2010, some of them are correct
and done now, some are I think incorrect. parseQuotedString does unicode
decoding and therefore needs to return QString.

Those two methods were not used outside kmime_header_parsing.cpp,
so better internalize all this.

The only exception was an unfinished non-automated test program,
adjusted accordingly.

Fix discarded content if no Content-Type
When replying to an email without any content type header,
the content was ignored (i.e. not quoted in the reply).

BUG: 426238

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