[lookandfeel] Fix switching to a different user session
Commit bcaf3886 removed the property
m in UserDelegate.qml, but it is still used by
userListCurrentModelData in SessionManagementScreen.qml at least.
This broke switching to an existing session via the "Switch User" button on the lock screen or the application launcher/menu, it just hung with an empty screen and this runtime error in the logs: file:///usr/share/plasma/look-and-feel/org.kde.breeze.desktop/contents/lockscreen/LockScreenUi.qml:444: TypeError: Cannot read property 'vtNumber' of undefined
Adding it back makes switching work again (and gets rid of the runtime error).