Do not replace nbsp characters when creating text/plain mime data

When copying from a QTextEdit with a non-breaking space character (nbsp,
U+00A0), the character will be replaced by an ordinary whitespace in the "text/plain" mime data.

This is different from other native text editors, at least on Windows; they preserve the character. It also creates an asymetry, because pasting such a character from another plain text editor works fine.

The reason for this behavior is that the default QTextEdit uses QTextDocumentFragment::toPlainText(), which in turn uses QTextDocument::toPlainText(). The latter one is documented to replace U+00A0, U+2029, and U+2028 characters.

A possible fix is to switch to QTextDocument::toRawText(), which avoids these conversions.