[applets/batterymonitor] Port inhibition message to Layouts and match notifications applet

Summary:
This makes it take up less vertical space in the header area and also match the similar
message in the notifications applet.

Test Plan:

Reviewers: VDG, Plasma, broulik, manueljlin, mart

Reviewed By: VDG, Plasma, mart

Subscribers: plasma-devel

Tags: Plasma

Maniphest Tasks: T10470

Differential Revision: https://phabricator.kde.org/D29315