Draft: applets/battery: show icon indicating performance mode/power save mode/inhibition

Indicate non-default power mode or sleep blocking with an icon for the applet:

• • When discharging, show this as an overlay to the battery icon.
  • When no batteries or plugged in, use the status icon as the applet icon (previously: without batteries present there was no indication of power mode at all.)
• • When discharging, show the overlay, but only when there is not already a percentage overlay as it otherwise won't fit (previously: while unplugged there was no power mode status indication shown at all).
  • When plugged in or no batteries, show the battery icon and status overlay as well (previously: the plugged battery icon would be replaced with a large version of the status icon).
• The icon is set no matter if the power mode/inhibition was invoked manually or requested by some application, and covers the case of inhibitions too (previously: the status indication only fired for profile holds, and also did not indicate inhibitions).

How to test:
In the battery applet, toggle sleep blocking.

Draft because:

• The overlay symbol is poorly visible; not sure how to improve this without making it large so that it ends up covering most of the battery icon. The current implementation is taken from https://invent.kde.org/plasma/plasma-workspace/-/blob/master/components/workspace/BadgeOverlay.qml. Maybe an overlay just doesn't work here and we should do with improving the conditions for replacing the battery icon with status icon instead?
• I couldn't find a good icon for "sleep blocked"; currently I use the speedometer ("speedometer") since the lighting symbol ("battery-profile-performance") is already used for performance mode, but "speedometer" is also already used for balanced power mode.

BUG: 471369
FIXED-IN: 6.0