[applets/notifications] Make configure button open the KCM

The notification applet's config window is blank, and only shows the auto-generated Shortcuts and About pages. This makes it not very useful, as typically the user will want to get to the KCM instead. Other applets with blank config windows (such as the Bluetooth and Networks applets) make the configure button open the KCM instead and nobody has complained about it, so let's do the same for the Notifications applet. This has two advantages:

1. Greater consistency among applets
2. The "Clear History" action always appears visibly in the header without getting pushed into a hamburger menu
3. A cleaner UI than the one merged in !837 (merged)

BUG: 433140 FIXED-IN: 5.22

cc @teams/vdg @ahiemstra