Fix encoding issue in qt_sinks

[neothenil]

QString::fromLatin1 doesn't handle non-latin characters properly. When applied to Chinese characters, the message appeared in QTextEdit looks like corrupted.

This PR fixes this issue.

[gabime]

Duplicate of the rejected #2836. Colors ranges must an addressed somehow

[neothenil]

Oh, I see. The problem is color ranges are byte indices, but they are passed to QString::left and QString::mid where the expected parameter is character index. How about converting color ranges from byte index to character index before passing to invoke_method_ like this commit does?

[gabime]

Not sure. This should be debugged. And anyway msg contents should not be changed directly since they may be used by other sinks

[neothenil]

For my use case, the modification works fine. The characters in QTextEdit are normal. The color ranges which were incorrect before the last commit also appears correct. And as long as the underlying string is UTF8-encoded, this modification should work well. In case the changed msg contents affect other sinks, I can make the converted color ranges local variables without changing msg directly.

[tt4g]

Sorry, a link to the source code was missing. See next comment.

[gabime]

This results in 3 QString::fromUtf8() per each log call. It is expensive and should be avoided if only ascii/latin1 is used.

Please add a bool flag is_utf8 to the constructor and perform the fromUtf8() only if set to true.

[neothenil]

The bool flag is_utf8 is added on your demand.

[gabime]

Thanks @neothenil and @tt4g . Merged.