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/tray.qml

@@ -10,8 +10,6 @@ BarWidget {
 
   property bool expanded: false
   property bool managePopupOpen: false
-  readonly property bool vertical: bar ? bar.vertical : false
-  readonly property int barSize: bar ? bar.barSize : Style.bar.sizeHorizontal
   readonly property color foreground: bar ? bar.foreground : Color.foreground
   readonly property string fontFamily: bar ? bar.fontFamily : Style.font.family
   readonly property var pinnedIds: Array.isArray(settings.pinned) ? settings.pinned : []