Summary: Current code only supports one player

Test Plan: Run two media sessions. Dbus inspection works fine, UI has some issues with values not updated, but out of scope for this diff

Reviewers: #kde_connect, apol

Reviewed By: #kde_connect, apol

Subscribers: apol, #kde_connect

Tags: #kde_connect

Differential Revision: https://phabricator.kde.org/D12546