Lift setting / vertical / barSize onto BarWidget

Six widgets (clock, daytime, keyboardLayout, lockKeys, activeWindow,
media) shipped identical copies of `function setting(name, fallback)`.
Eight widgets reimplemented `readonly property bool vertical`, and
three reimplemented `readonly property int barSize`. Move all three
onto the BarWidget base so widgets read the same wiring everyone else
gets for free.

While in here, drop the dead `setting()` declaration on media.qml — it
was defined but never called.

shell/plugins/bar/widgets/systemStats.qml

@@ -116,8 +116,7 @@ BarWidget {
     onTriggered: root.refresh()
   }
 
-  readonly property bool vertical: bar ? bar.vertical : false
-  readonly property color statColor: bar ? bar.foreground : "#cacccc"
+  readonly property color statColor: bar ? bar.foreground : Color.foreground
 
   implicitWidth: button.implicitWidth
   implicitHeight: button.implicitHeight