[PlasmaComponents3] Honor icon.[name|source] property

Since a checkbox is a subclass of AbstractButton, it can be given an
icon. QWidgets checkboxes will faithfully display this icon, but
PlasmaComponents3 ones will not. They should. :)

I will submit a similar change for the QQC2 checkbox style
to make them consistent.

src/declarativeimports/plasmacomponents3/CheckBox.qml

@@ -18,6 +18,7 @@
  */
 
 import QtQuick 2.6
+import QtQuick.Layouts 1.3
 import QtQuick.Templates @QQC2_VERSION@ as T
 import QtQuick.Controls @QQC2_VERSION@
 import org.kde.plasma.core 2.0 as PlasmaCore
@@ -32,8 +33,9 @@ T.CheckBox {
                                       indicator ? indicator.implicitHeight : 0) + topPadding + bottomPadding)
     baselineOffset: contentItem.y + contentItem.baselineOffset
 
-    padding: 1
-    spacing: Math.round(units.gridUnit / 8)
+    leftPadding: control.indicator && !control.mirrored ? control.indicator.width + control.spacing : 0
+    rightPadding: control.indicator && control.mirrored ? control.indicator.width + control.spacing : 0
+    spacing: units.smallSpacing
 
     hoverEnabled: true
 
@@ -47,16 +49,26 @@ T.CheckBox {
         control: control
     }
 
-    contentItem: Label {
-        leftPadding: control.indicator && !control.mirrored ? control.indicator.width + control.spacing : 0
-        rightPadding: control.indicator && control.mirrored ? control.indicator.width + control.spacing : 0
+    contentItem: RowLayout {
         opacity: control.enabled ? 1 : 0.6
-        text: control.text
-        font: control.font
-        color: PlasmaCore.ColorScope.textColor
-        elide: Text.ElideRight
-        visible: control.text
-        horizontalAlignment: Text.AlignLeft
-        verticalAlignment: Text.AlignVCenter
+        spacing: units.smallSpacing
+
+        PlasmaCore.IconItem {
+            source: control.icon.name || control.icon.source
+            visible: source.length > 0
+
+            implicitWidth: units.iconSizes.smallMedium
+            implicitHeight: units.iconSizes.smallMedium
+        }
+
+        Label {
+            text: control.text
+            font: control.font
+            color: PlasmaCore.ColorScope.textColor
+            elide: Text.ElideRight
+            visible: control.text
+            horizontalAlignment: Text.AlignLeft
+            verticalAlignment: Text.AlignVCenter
+        }
     }
 }

tests/components/checkbox3.qml

@@ -17,6 +17,21 @@ ComponentBase {
             text: "Some awesome checkbox"
         }
 
+        PlasmaComponents.Label {
+            text: "icon"
+        }
+        PlasmaComponents.CheckBox {
+            icon.name: "start-here-kde-plasma"
+        }
+
+        PlasmaComponents.Label {
+            text: "text plus icon"
+        }
+        PlasmaComponents.CheckBox {
+            text: "text"
+            icon.name: "start-here-kde-plasma"
+        }
+
         PlasmaComponents.Label {
             text: "focus"
         }