Support pixel-by-pixel scrolling in QPlainTextEdit

For performance reasons, QPlainTextEdit only supports scrolling paragraph-by-paragraph, not pixel-by-pixel the way most QT views do.

Several KDE applications (KEdit, Kate, Konsole) use QPlainTextEdit for its high performance, but we would like to implement pixel-by-pixel scrolling to improve the consistency of the user's scroll experience. We are willing to accept the performance hit caused by using pixel-by-pixel scrolling, but don't need tables or embedded frames, so we would like to avoid using QTextEdit instead, as we would inherit its worse performance from supporting those features that we don't need. All we need is for QPlainTextEdit to optionally support pixel-by-pixel scrolling. Thanks!