Relay Clearing Histogram to Server

As the server also keeps track of the events, clearing the histogram
should also be done there.

src/clientgui.cpp

@@ -259,6 +259,8 @@ void ClientGui::clearHistogram()
     _histo->ResetStats();
     _rootcanvas.getCanvas()->Modified();
     _rootcanvas.getCanvas()->Update();
+
+    sendSimpleActiontoServer(Protocol::Action::clearHistogram);
 }
 
 void ClientGui::showEvent(QShowEvent* event)