Support multiple players in MprisRemote

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