[applets/batterymonitor] Overhaul configuration and actions

Right now the applet puts the entries to open the KCM and Energy Info page in a hamburger menu, while the Configure button opens the applet's own settings window which has only one item in it. This emphasizes the less important thing, while de-emphasizing the more important thing.

We can solve this by making the applet's configure button open the KCM instead of the applet configuration window. This makes the configuration window inaccessible, so we delete it and move its one setting into the hamburger menu along with "Show Energy Information". This is okay as these two are lesser-used settings.

Overall this should make the actions easier to access and use, and more relevant to the user.

BUG: 407560 FIXED-IN: 5.22

cc @teams/vdg