Add unit tests for PagePool/PagePoolAction

Summary:
In addition to the new unit tests a fix has been added for
initialProperties being ignored when they are already
specified in the QML Component file.

Test Plan: See the tst_pagepool unit test included.

Reviewers: Kirigami, mart

Reviewed By: Kirigami, mart

Subscribers: plasma-devel, mart

Tags: Kirigami

Differential Revision: https://phabricator.kde.org/D28690