Add “Send Histogram Update” Command

This simple command asks the server to send the full content of the
histogram.

src/command.cpp

@@ -82,6 +82,7 @@ Command Command::fromString(const QString& source)
         case Action::fillHistogram:
             return parseFillHistogram(rootobj);
         case Action::sendHistogramUpdate:
+            return Command(action, Arguments(), token);
         case Action::moveTargetCloser:
         case Action::moveTargetFarther:
         case Action::openVacuumValve: