ApplicationModel takes ownership of PageModels

Summary:
The ownership was misguided before and stayed with whoever create the
page (likely the available pages model). It caused two issues:

• constant memory increase since the available pages model didn't know when a page model wasn't necessary;
• sometimes crash on exit.

Now ApplicationModel forcibly takes ownership and manage this sub-model
using a shared pointer like the other ones. Way more consistent and
predictible.

Reviewers: bensi, franckarrecot

Differential Revision: https://phabricator.kde.org/D597