Show captive portal login page in a dedicated window

This streamlines the entire process and separates it more clearly from whatever else might be going on on the users machine and browser. It also gives us the opportunity to give us more context to what is happening (for example with the window title). A more niche case are some devices (such as the steamdeck) that might come without having a browser preinstalled leaving the user in an impossible situation requiring a browser to gain network access to be able to install a browser. If for whatever reason the authentication page might not work in QWebEngine the current page can still be opened in a browser via a button.

This is how it looks like initially (with kde.org instead of a captive portal login page for illustration):

After the user authenticated and now has network access: