Move power management checkbox to the top

Summary:
BUG: 345940

Move the "Enable Power Management" checkbox to the top of the window to match other applets (bluetooth, network)

Reviewers: Plasma: Workspaces, broulik, mck182, davidedmundson, mart

Reviewed By: Plasma: Workspaces, davidedmundson, mart

Subscribers: mart, jensreuterberg, davidedmundson, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D8607