QmlDesigner: Adding tooltip for canvas color

Task-number: QTCREATORBUG-14819
Change-Id: I0929f899e84e6bc5415d8c699fc4df53a2fe561b
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@qt.io>

src/plugins/qmldesigner/components/formeditor/backgroundaction.cpp

@@ -67,6 +67,7 @@ QWidget *BackgroundAction::createWidget(QWidget *parent)
     connect(comboBox, SIGNAL(currentIndexChanged(int)), SLOT(emitBackgroundChanged(int)));
 
     comboBox->setProperty("hideborder", true);
+    comboBox->setToolTip(tr("Set the color of the canvas."));
     return comboBox;
 }