In KCommonDecoration the OnAllDesktops button gets hidden or shown
depending on the number of desktops. For that KDecoration is extended
by a new property which delegates to the bridge to return whether
onAllDesktops is available. In KWin Core this is implemented using
the number of desktops.
FEATURE: 321611
FIXED-IN: 5.0.0
REVIEW: 116076