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/clock.qml

@@ -9,11 +9,6 @@ BarWidget {
 
   property bool alt: false
 
-  function setting(name, fallback) {
-    var value = settings ? settings[name] : undefined
-    return value === undefined || value === null ? fallback : value
-  }
-
   function label() {
     if (alt) return Qt.formatDateTime(clock.date, String(setting("formatAlt", "dd MMMM 'W'ww yyyy")))
     if (bar && bar.vertical) return Qt.formatDateTime(clock.date, String(setting("verticalFormat", "HH\n—\nmm")))