Add toPlainText method to QQuickText

Provide a way to check the plain text characters of qquicktext, excluding characters that are markup. Ideally exposing something like QTextDocument::toPlainText. Our use case is to truncate certain text fields based on a max character count.

A not so nice workaround is to use a parallel QTextDocument instance, but this would only yield correct results in case of RichText, not in case of StyledText:

doc = QTextDocument()
doc.setHtml(markupText)
text = doc.toPlainText()