Dropdown lists (combobox select menus) in *Choose a Layout* window does not react on keypresses

#### Expected behaviour
User is able to select needed list items by clicking corresponding button on keyboard to scroll the list to the needed first letter.

#### Actual behaviour
User have to use mouse or keyboard to select needed list item. This process is long and time-consuming.

#### Steps to reproduce the behaviour
1. Open `mate-keyboard-preferences`
1. Switch to *Layouts* tab
1. Click *Add*
* On the *By country* tab
     - set focus on *Country* and try to choose *Russian Federation* from it by pressing <kbd>r</kbd> one or several times
     - set focus on *Variants* and try to choose *Russian* from it by pressing <kbd>r</kbd> one or several times

* On the *By language* tab
     - set focus on *Language* and try to choose *Russian* from it by pressing <kbd>r</kbd> one or several times
     - set focus on *Variants* and try to choose *Russian* from it by pressing <kbd>r</kbd> one or several times

(see screencast from attachment)

ProblemType: Bug
DistroRelease: Ubuntu 19.10
Package: mate-control-center 1.22.1-0ubuntu2.1
ProcVersionSignature: Ubuntu 5.2.0-8.9-generic 5.2.0
Uname: Linux 5.2.0-8-generic x86_64
ApportVersion: 2.20.11-0ubuntu5
Architecture: amd64
CurrentDesktop: MATE
Date: Thu Jul 18 19:44:25 2019
InstallationDate: Installed on 2019-07-03 (14 days ago)
InstallationMedia: Ubuntu-MATE 19.10 "Eoan Ermine" - Alpha amd64 (20190703)
SourcePackage: mate-control-center
UpgradeStatus: No upgrade log present (probably fresh install)