Hide the OnAllDesktops button if there is only one virtual desktop

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