Use native QML icon support

app/gui/NavigableToolButton.qml

@@ -7,17 +7,9 @@ ToolButton {
 
     activeFocusOnTab: true
 
-    // FIXME: We're using an Image here rather than icon.source because
-    // icons don't work on Qt 5.9 LTS.
-    Image {
-        id: image
-        source: iconSource
-        anchors.centerIn: parent.background
-        sourceSize {
-            width: parent.background.width * 1.10
-            height: parent.background.height * 1.10
-        }
-    }
+    icon.source: iconSource
+    icon.width: background.width * 1.10
+    icon.height: background.height * 1.10
 
     // This determines the size of the Material highlight. We increase it
     // from the default because we use larger than normal icons for TV readability.