[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: {F8274308}

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