Rename meta-weston-launch to meta-launcher

We're going to replace this with a login1 implementation soon enough.
launcher: Fix up includes
launcher: Move activate_session abstraction into the launcher code

login1 has a separate ActivateSession request, so put it here so
that we don't have to modify any backend code.
launcher: Unconditionally set the FD

We only open weston-launch if we're in the native backend,
so we don't need to check here.
launcher: Fix indentation
launcher: Rename methods a bit