Move AboutKDE to public namespace, remove button from AboutPage
Reasoning
This has just caused more issues, because we make some assumptions about the page stack. Plasma welcome has issues due to it's unique structure, and has requested to be able to open the "About KDE" window separately. Other applications break like NeoChat (BUG: 464719) because there's some inherent assumptions about the application structure in AboutPage caused by the MR that introduced this feature (!70 (merged))
Solution
We could set up a property in AboutPage
to decide how to open the "About KDE" window, etc but I think
that's just weird and complex - all of this just to open a page! Instead, let's move it into a public namespace and let
applications handle how to display and open "About KDE" themselves. The reason why we made it private in the
first place I think is that it would be uniformly distributed to every Kirigami-Addons application
that uses the AboutPage, but I think it would be better if we find another way for now.