[applets/batterymonitor] PowerManagement: Fix header's height

"Fix" as in "repair and make it right". This commit actually makes
 header dynamic as it was always supposed to be. Wrongly fixed height
 with dummy empty padding at the bottom was the result of incorrectly
 applied anchors within Container[1]'s contentChildren.

 [1]: PlasmoidHeading is a TabBar which inherits from Container and Control.