refactor(ui): One QML file per screen; the HDMI boost's setup screens and backend commands removed

ui/qml/Bar.qml

@@ -0,0 +1,7 @@
+// The bar at the bottom of a screen, for its buttons.
+import QtQuick
+
+Rectangle {
+    anchors.bottom: parent.bottom; width: parent.width; height: 64; color: Theme.bar
+    Rectangle { width: parent.width; height: 1; color: Theme.line }
+}