"repeat" checkbox gets out of sync with actual repeat state

If you use an external application to ask totem to start/stop repeating (via e.g. `playerctl loop -p totem None` or `playerctl loop -p totem Playlist`), then the "Repeat" checkbox in the UI changes state, but totem doesn't actually start/stop repeating the track.

Concretely:

- start totem playing a file
- tick the "Repeat" checkbox
- run `playerctl loop -p totem None`
- observe that the "Repeat" checkbox is no longer checked
- observe that totem continues to repeat the music

totem version: 3.26.0-0ubuntu6.2
playerctl (https://github.com/altdesktop/playerctl) version: 2.2.1

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: totem 3.26.0-0ubuntu6.2
ProcVersionSignature: Ubuntu 4.15.0-112.113-generic 4.15.18
Uname: Linux 4.15.0-112-generic x86_64
NonfreeKernelModules: nvidia_modeset nvidia
ApportVersion: 2.20.9-0ubuntu7.16
Architecture: amd64
CurrentDesktop: Unity7
Date: Tue Aug 25 14:58:34 2020
InstallationDate: Installed on 2018-08-13 (742 days ago)
InstallationMedia: Ubuntu 18.04.1 LTS "Bionic Beaver" - Release amd64 (20180725)
SourcePackage: totem
UpgradeStatus: No upgrade log present (probably fresh install)