mate-keyboard-properties doesn't allow me to add more than 4 keyboard layouts

[Aminda Suomalainen]

After I have added four keyboard layouts, the add button becomes grayed out. I have a hobby of learning languages and the EU has a goal of everyone studying their native language and two other languages which can very easily introduce the need to use more layouts.

I also reported this at https://github.com/mate-desktop/mate-control-center/issues/364 where I understood them to also wish for downstream report so I am reporting this just in case.

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: mate-control-center 1.20.2-2ubuntu1
ProcVersionSignature: Ubuntu 4.15.0-24.26-generic 4.15.18
Uname: Linux 4.15.0-24-generic x86_64
NonfreeKernelModules: nvidia_modeset nvidia
ApportVersion: 2.20.9-0ubuntu7.2
Architecture: amd64
CurrentDesktop: MATE
Date: Wed Jul 18 12:05:51 2018
InstallationDate: Installed on 2018-04-30 (78 days ago)
InstallationMedia: Ubuntu-MATE 18.04 LTS "Bionic Beaver" - Release amd64 (20180426)
SourcePackage: mate-control-center
UpgradeStatus: No upgrade log present (probably fresh install)