PlanToolBarIndicators: Use more descriptive label for distance to WP

The current implementation displays two different distances by the same
name. One of the distances is actually the distance to the previous
waypoint. For the user its "hard" to tell which one is which. I try to
resolve this with a more descriptive label for the distance to the
previous waypoint.

An alternative is maybe to rename the other distance to total distance
or total mission distance.

src/PlanView/PlanToolBarIndicators.qml

@@ -158,7 +158,7 @@ Item {
 
             Item { width: 1; height: 1 }
 
-            QGCLabel { text: qsTr("Distance:"); font.pointSize: _dataFontSize; }
+            QGCLabel { text: qsTr("Dist prev WP:"); font.pointSize: _dataFontSize; }
             QGCLabel {
                 text:                   _distanceText
                 font.pointSize:         _dataFontSize