PreferencesWindow: code style, Gtk4 prep

[danirabbit]

Clean up and Gtk 4 prep

• Use default size so we don't hardcode a minimum size
• Granite.Dialog already hides the close button, so this allows closing with esc
• Can't set our own window position in Gtk4, should center parent anyways
• Remove a lot of unnecessary properties from the switcher. Halign does what we need
• Box instead of Grid
• Use add_button from dialog instead of creating another action area
• Remove unnecessary cast