Port Kickoff to shared view component using single-MouseArea pattern

Summary:
This:
* Moves MouseArea out of the item delegate and uses a single MouseArea
  per view.
* Ports all the pages to using a shared view component.

Aside from saving a lot of QObjects, using a single MouseArea also
has the advantage that MouseArea.positionChanged isn't fired when
QQuickWindow synthesizes a hover event per frame, unlike MouseArea
.containsMouse. This fixes a user-reported bug in the search page,
and the same pattern is used by various other UIs we have for
similar reasons.

BUG:397693

Reviewers: #plasma, ngraham, davidedmundson

Reviewed By: #plasma, davidedmundson

Subscribers: davidedmundson, ngraham, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D15855