Fix Slider vertical implicitWidth and a binding loop

The implicitWidth was using the horizontal implicitWidth for vertical
sliders, which is way too wide.
The binding loops was fixed just by specifying an id for the implicitWidth value used for contentWidth.

org.kde.desktop/Slider.qml

@@ -16,8 +16,8 @@ T.Slider {
     @DISABLE_UNDER_QQC2_2_4@ palette: Kirigami.Theme.palette
     Kirigami.Theme.colorSet: Kirigami.Theme.Button
 
-    implicitWidth: Kirigami.Units.gridUnit * 12
-    implicitHeight: background.implicitHeight
+    implicitWidth: background.horizontal ? Kirigami.Units.gridUnit * 12 : background.implicitWidth
+    implicitHeight: background.horizontal ? background.implicitHeight : Kirigami.Units.gridUnit * 12
 
     hoverEnabled: true
     LayoutMirroring.enabled: Qt.application.layoutDirection === Qt.RightToLeft
@@ -31,8 +31,8 @@ T.Slider {
         elementType: "slider"
         sunken: controlRoot.pressed
         implicitWidth: 200
-        contentHeight: horizontal ? 22 : implicitWidth
-        contentWidth: horizontal ? implicitWidth : 22
+        contentWidth: horizontal ? controlRoot.implicitWidth : 22
+        contentHeight: horizontal ? 22 : controlRoot.implicitHeight
 
         maximum: controlRoot.to*100
         minimum: controlRoot.from*100