split off an AboutItem from AboutPage

This creates a separate AboutItem out of the AboutPage, which is not embedded in a Kirigami Page and, more importantly, does not implicitly rely on a Kirigami ApplicationWindow to be present. This makes this component more easily usable by QML desktop applications. With this, I can remove the rather heavy KXMLGui dependency of Skanpage by displaying the about data with this new component. Implementation is at https://invent.kde.org/astippich/skanpage/-/commits/about

It is possible to just embed the AboutPage in a QML window and then show it. However, the AboutPage calls applicationWindow() to display the license, which obviously fails when not using an Kirigami ApplicationWindow as the main window. If one is using a Kirigami ApplicationWindow, the OverlaySheet is displayed below the separate window, which is also incorrect.

Let me know what you think.