Update screenshots in documentation

Moved unresolved from QTBUG-63248:

• QMessageBox uses old macOS aqua looks http://doc.qt.io/qt-5/qmessagebox.html
• QTabWidget has some kind of win98 looks : http://doc.qt.io/qt-5/qtabwidget.html
• Screenshot is miniscule: http://doc.qt.io/qt-5/examples-qtassistant.html
• this one has no image: http://doc.qt.io/qt-5/qtassistant-remotecontrol-example.html
• ...and the link to the image at bottom is a 404 : Caused issue QTBUG-69821 . Must be resolved there.
• this one has no image either : http://doc.qt.io/qt-5/qthelp-contextsensitivehelp-example.html
• this one has an image which looks fairly aliased : http://doc.qt.io/qt-5/qtdesigner-taskmenuextension-example.html

Frankly, looking at them all, it's all over the place ! shouldn't there be a CI automation script that would take the screenshots automatically on new qt releases ? Because that's what people will see when they use it.