Provide option to hide menu bar for Ksysguard

Summary: BUG: 395349

CHANGELOG: System Monitor now has the ability to hide its menu bar

Reviewers: ngraham, #plasma, #frameworks

Reviewed By: ngraham, sitter

Subscribers: broulik, sitter, acrouthamel, ngraham, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D15644