systemd takes care of setting and dropping master permissions when
sending PauseDevice and ResumeDevice signals.
When the ResumeDevice signal is received, the relevant drm device should
already have master permissions set up.
On the other hand, when the active property changes, there's still a
chance that systemd haven't granted drm master permissions to us.
At the moment, the session code is far from being extensible. If we
decide to add support for libseatd, it will be a challenging task with
the current design of session management code. The goal of this
refactoring is to fix that.
Another motivation behind this change is to prepare session related code
for upstreaming to kwayland-server where it belongs.
