Kicker backend changes for Simple Menu.

Summary:
This does three changes needed to support the Simple Menu UI:

* Support for pagination with configurable page size and related
  API naming changes.

* Make showing the Power / Session top-level category optional.

* Emit a signal on wheel move events in WheelInterceptor.

As well as adding inheritance from QQmlParserStatus to RootModel
to avoid startup churn.

Reviewers: #plasma, mart

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D3655