backends: add `isSessionActive`
`isSessionActive` is a boolean that is true if the session is active, i.e. currently in the foreground. Doc: https://www.freedesktop.org/software/systemd/man/org.freedesktop.login1.html
`isSessionActive` is a boolean that is true if the session is active, i.e. currently in the foreground. Doc: https://www.freedesktop.org/software/systemd/man/org.freedesktop.login1.html
mentioned in commit fc4a6488