[WIP] Add qmldemo skin

res/skins/qmldemo/MixxxSlider.qml

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+import QtQuick 2.12
+import QtQuick.Controls 2.12
+import Mixxx 1.0
+
+Item {
+    id: root
+    property alias group: control.group
+    property alias key: control.key
+
+    Slider {
+        id: slider
+        orientation: Qt.Vertical
+        value: control.parameter
+        wheelEnabled: true
+
+        Control {
+            id: control
+            parameter: slider.value
+        }
+
+        handle: Image {
+            source: "../LateNight/palemoon/sliders/knob_volume_deck.svg"
+            width: 42
+            height: 19
+            x: slider.leftPadding + slider.availableWidth / 2 - width / 2
+            y: slider.visualPosition * (slider.height - height)
+        }
+
+        background: Image {
+            source: "../LateNight/palemoon/sliders/slider_volume_deck.svg"
+        }
+    }
+}

res/skins/qmldemo/MixxxToggleButton.qml

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+import QtQuick 2.12
+import QtQuick.Controls 2.12
+import Mixxx 1.0
+
+Item {
+    id: root
+    property alias text: label.text
+    property alias group: control.group
+    property alias key: control.key
+
+    Button {
+        id: button
+        highlighted: control.value
+
+        Control {
+            id: control
+        }
+
+        onPressed: control.value ? control.value = 0.0 : control.value = 1.0
+
+        contentItem: Text {
+            id: label
+        }
+
+        background: Rectangle {
+            color: button.highlighted ? "#404040" : "#303030"
+        }
+    }
+}

res/skins/qmldemo/main.qml

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+import QtQuick 2.15
+import QtQuick.Controls 2.15
+import QtQuick.Layouts 1.11
+
+Rectangle {
+    id: window
+    width: 1920
+    height: 1080
+    color: "#202020"
+    visible: true
+
+    RowLayout {
+        anchors.fill: parent
+        spacing: 6
+
+        MixxxSlider {
+            width: 34
+            height: 100
+            group: "[Channel1]"
+            key: "volume"
+        }
+
+        MixxxToggleButton {
+            width: 34
+            height: 22
+            text: "Play"
+            group: "[Channel1]"
+            key: "play"
+        }
+    }
+}

res/skins/qmldemo/skin.ini

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+[Skin]
+description = "A QML example skin."
+min_pixel_width = 1920
+min_pixel_height = 1080