OP-975 give to empty image a meaningful name

ground/openpilotgcs/src/plugins/config/configgadget.qrc

@@ -36,7 +36,7 @@
         <file>images/calibration/plane-right.png</file>
         <file>images/calibration/plane-up.png</file>
         <file>images/calibration/plane-horizontal-rotated.png</file>
-        <file>images/calibration/snow.png</file>
+        <file>images/calibration/empty.png</file>
         <file>images/calibration/plane-down.png</file>
         <file>images/calibration/revo-down.png</file>
         <file>images/calibration/revo-flip.png</file>

ground/openpilotgcs/src/plugins/config/configrevowidget.cpp

@@ -81,7 +81,7 @@ ConfigRevoWidget::ConfigRevoWidget(QWidget *parent) :
     m_ui->calibrationVisualHelp->setScene(new QGraphicsScene(this));
     m_ui->calibrationVisualHelp->setRenderHint(QPainter::HighQualityAntialiasing, true);
     m_ui->calibrationVisualHelp->setRenderHint(QPainter::SmoothPixmapTransform, true);
-    displayVisualHelp("snow");
+    displayVisualHelp("empty");
     // Must set up the UI (above) before setting up the UAVO mappings or refreshWidgetValues
     // will be dealing with some null pointers
     addUAVObject("RevoCalibration");