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/spacer.qml

@@ -5,11 +5,9 @@ BarWidget {
   id: root
   moduleName: "spacer"
 
-
-  readonly property bool vertical: bar ? bar.vertical : false
   readonly property int span: settings && settings.size !== undefined ? Number(settings.size) : 12
 
-  implicitWidth: vertical ? (bar ? bar.barSize : 28) : span
-  implicitHeight: vertical ? span : (bar ? bar.barSize : 26)
+  implicitWidth: vertical ? barSize : span
+  implicitHeight: vertical ? span : barSize
   visible: span > 0
 }