[Task Manager] Don't show redundant "Actions" header in context menu with nothing but actions in it

Summary:
Previously, the Actions header was always shown, even when the context menu
consisted of nothing but actions. This is unnecessary since a menu's normal state is
to show nothing but actions.

With this patch, the Actions header is only shown for context menus that have
more than one section.

Test Plan:
Context menu with only actions:

Context menu with nore than just actions:

Reviewers: broulik, Plasma, hein

Reviewed By: Plasma, hein

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D20846