Add a explicit page type check in AbstractApplictionHeader

Pushing a non-page type in the page row, will emit a error by the QtQuick runtime. Now there's an explicit type check before that happens.

Context

When opening Kirigami Add-ons settings pages, the application emits an error like this:

file:///usr/lib64/qml/org/kde/kirigami/templates/AbstractApplicationHeader.qml:40:5: Unable to assign SettingsPage_QMLTYPE_346 to Page_QMLTYPE_192

This is because the base type of SettingsPage is another PageRow, but if we add an explicit as check that helps prevent the error. Not sure if this is the best way to solve the error, but the type check is a good idea anyway?

@carlschwan