Implement native system settings GetColour in wxQT

This was causing incorrect background in paint events and other drawing issues.
Add wxNORMAL_FONT in native system setting
Fix wxQT painting to honor background style

Note that although this works better, the internal m_qtPaintBuffer still causes some issues and is inefficient (not needed as Qt already do a double buffering for widget painting)
Also, QWidget::setBackgroundRole et.al. should be enforced too (and tested), to properly handle transparent and system backgrounds