Summary:
Not sure what operator+ overload has been used exactly for the int,
in any case it does not work as intended and needs e.g. an explicit
QString::number() invocation.

Also start with number 2 for duplicated instances.

Test Plan:
Start multiple instances of an MPRIS player (e.g. Gwenview). Before the
second instance would get labelled with "Name []", with this patch it is
labelled with "Name [2]".

Reviewers: #kde_connect, mtijink

Reviewed By: #kde_connect, mtijink

Subscribers: mtijink, nicolasfella

Tags: #kde_connect

Differential Revision: https://phabricator.kde.org/D11411