[icons] ValueError: Not an icon name: arrow_keys
When opening a PlotWindow then clicking on the Options button to try to display the legend, I get the following error message : 

```
File "/users/knobel/.local/lib/python2.7/site-packages/silx/gui/plot/PlotWindow.py", line 358, in _controlButtonClicked
    controlMenu.addAction(self.panWithArrowKeysAction)
  File "/users/knobel/.local/lib/python2.7/site-packages/silx/gui/plot/PlotWindow.py", line 317, in panWithArrowKeysAction
    self._panWithArrowKeysAction = PanWithArrowKeysAction(self)
  File "/users/knobel/.local/lib/python2.7/site-packages/silx/gui/plot/PlotActions.py", line 945, in __init__
    checkable=True, parent=parent)
  File "/users/knobel/.local/lib/python2.7/site-packages/silx/gui/plot/PlotActions.py", line 103, in __init__
    icon = icons.getQIcon(icon)
  File "/users/knobel/.local/lib/python2.7/site-packages/silx/gui/icons.py", line 168, in getQIcon
    qfile = getQFile(name)
  File "/users/knobel/.local/lib/python2.7/site-packages/silx/gui/icons.py", line 217, in getQFile
    raise ValueError('Not an icon name: %s' % name)
ValueError: Not an icon name: arrow_keys

```

diff --git a/silx/gui/plot/PlotActions.py b/silx/gui/plot/PlotActions.py
index db0ff906a..b742b9e96 100644
--- a/silx/gui/plot/PlotActions.py
+++ b/silx/gui/plot/PlotActions.py
@@ -937,7 +937,7 @@ class PanWithArrowKeysAction(_PlotAction):
     def __init__(self, plot, parent=None):
 
         super(PanWithArrowKeysAction, self).__init__(
-            plot, icon='arrow_keys', text='Pan with arrow keys',
+            plot, icon='arrow-keys', text='Pan with arrow keys',
             tooltip='Enable pan with arrow keys when checked',
             triggered=self._actionTriggered,
             checkable=True, parent=parent)
['silx/gui/plot/PlotActions.py']