Release 6.20.0: Metadata for Config Cascade system

[changerequest]

The file
conf/meta/ui.yml that is installed at /opt/openexchange/etc/groupware/meta/ui.yml
contains the necessary metadata to declare the two new ui properties
ui/global/toolbar/mode/value
ui/global/windows/mode/value
as writable. This has to be done to make them user changeable.