Support services on flatpak

Summary:
We have special casing for flatpak and Kate is mimicking KDBusService
only to some extent. This change helps kate support multiple flatpak instances
as well.

Test Plan: Built and made it work locally

Reviewers: Kate, cullmann

Reviewed By: Kate, cullmann

Subscribers: cullmann, aacid, kwrite-devel

Tags: Kate

Differential Revision: https://phabricator.kde.org/D27208