[plot] Adding manually a non-visible item to a plot make this item unusable
Diagnosed together with @t20100 

This not works
```
        self.__toolTipMarker = Marker()
        self.__toolTipMarker._setLegend("marker-tooltip")
        self.__toolTipMarker.setColor("pink")
        self.__toolTipMarker.setSymbol("+")
        self.__toolTipMarker.setSymbolSize(10)
        self.__toolTipMarker.setVisible(False)
        self.__plot._add(self.__toolTipMarker)
```
This works
```
        self.__toolTipMarker = Marker()
        self.__toolTipMarker._setLegend("marker-tooltip")
        self.__toolTipMarker.setColor("pink")
        self.__toolTipMarker.setSymbol("+")
        self.__toolTipMarker.setSymbolSize(10)
        self.__plot._add(self.__toolTipMarker)
        self.__toolTipMarker.setVisible(False)
```

diff --git a/silx/gui/plot/PlotWidget.py b/silx/gui/plot/PlotWidget.py
index 49e444a47..e47249e2e 100755
--- a/silx/gui/plot/PlotWidget.py
+++ b/silx/gui/plot/PlotWidget.py
@@ -624,8 +624,7 @@ class PlotWidget(qt.QMainWindow):
         # Add item to plot
         self._content[key] = item
         item._setPlot(self)
-        if item.isVisible():
-            self._itemRequiresUpdate(item)
+        self._itemRequiresUpdate(item)
         if isinstance(item, items.DATA_ITEMS):
             self._invalidateDataRange()  # TODO handle this automatically
 
['silx/gui/plot/PlotWidget.py']