Massively simplify the class DesktopPortal

Summary:
Only one QObject can be registered on a DBus path...but that object can
have
multiple adaptors, which is how it's structured anyway.

It's considerably easier than doing everything manually through a
QDBusVirtualObject

Test Plan:
Checked all ifaces registered in qdbusviewer
Manually ran AddNotification, debug appeared in xdg-desktop-portal and a
notification appeared

Reviewers: #plasma, jgrulich

Reviewed By: jgrulich

Subscribers: apol, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D6145