Status notifier shows appindicators in random order.

To reproduce:

    1. run several programs that create appindicators.
    2. add multiple sn-plugin to the panel.

Result:

    Each sn-plugin shows the appindicators in a different order.

Expected result:

    The order should be deterministic, ie the order is not important but it should always be the same given the same appindicators are running.

Screenshot attached.

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: xfce4-statusnotifier-plugin 0.2.1-0ubuntu1
ProcVersionSignature: Ubuntu 4.15.0-12.13-generic 4.15.7
Uname: Linux 4.15.0-12-generic x86_64
ApportVersion: 2.20.8-0ubuntu10
Architecture: amd64
CurrentDesktop: XFCE
Date: Sun Mar 18 00:32:59 2018
InstallationDate: Installed on 2018-03-15 (2 days ago)
InstallationMedia: Xubuntu 18.04 LTS "Bionic Beaver" - Alpha amd64 (20180315)
SourcePackage: xfce4-statusnotifier-plugin
UpgradeStatus: No upgrade log present (probably fresh install)