[Approved] Split battery applet into two: "Power Management" and "Brightness"

This was raised here https://bugs.kde.org/show_bug.cgi?id=424283 but I would like to get some more opinions on it from people who are involved in the code. Since we're going to want to dig the applet over to port away from data engines soon anyway this would be a natural breaking point.

Reasons in favor imo:

1. There is no strong reason the two should be in the same applet to begin with; surely brightness affects power usage, but so does wifi, bluetooth and just about anything you do on your computer, and it's not relevant for external screens either, so it seems a little random to put these two in the same place.
2. The applet is getting full: We now have block inhibiting, screen brightness, keyboard brightness, power profile, battery status for the laptop, battery health for the laptop, battery status for bluetooth devices - this already fills the default applet popup size, and we are planning to include more brightness sliders for both internal and external screens, and potentially backlight color setting for RGB keyboard, so the applet would have to be scrolled or manually resized to access all controls which is not ideal.

Resaons against:

1. More tray icons, which some users don't like

Even as someone who doesn't like many tray icons myself I think a split makes sense.

@teams/usability @teams/vdg @fusionfuture @ratijas @ngraham