Set background color for QPdfView

Currently, QPdfView only allows for a white background. Most pdf viewers allow you to change the background color (for accessibility). I suggest adding a simple setter and getter function to QPdfView to change the background color.

QPdfView::setBackgroundColor(const QColor &)

QColor QPdfView::getBackgroundColor()

Other than adding an extra QColor backgroundColor to the class, and the getter and setter functions, it would only require changing one line in paintEvent.

QPdfView::paintEvent
...

painter.fillRect(pageGeometry, Qt::white);