Release 6.20.0: New configuration file ui.properties introduced

[changerequest]

/opt/open-xchange/etc/groupware/settings/ui.properties:

# UI: Window Manager setting
# Possible values: embedded OR popups
ui/global/windows/mode/value=embedded

# Allow user to configure the window manager
# Possible values: true OR false
ui/global/windows/mode/configurable=true

# UI: Toolbar type
# Possible values: tabbased OR simple
ui/global/toolbar/mode/value=tabbased

# Allow user to configure the toolbar type
# Possible values: true OR false
ui/global/toolbar/mode/configurable=true