Clean up interaction/code duplication between QDialog and subclasses for native dialogs

QDialog's various subclasses (QMessageBox, QFileDialog, QFontDialog, QColorDialog, etc), all override large parts of the setVisible machinery, with various levels of code duplication resulting in slightly different behaviors.

This is especially problematic for native dialogs, ie canBeNativeDialog, setNativeDialogVisible, and how those are interacting with Qt::WA_DontShowOnScreen, Qt::WA_WState_Hidden, and Qt::WA_WState_Visible.