IEngineAddon.engineStarted method called 2 times on start

For a given language specific IEngineAddon (ie. declared in XDSML_Definition tag of the plugin.xml of the xdsml project)

The addon method engineStarted is called 2 times instead of one.

(I haven't checked with engine addon that is not language specific)

To reproduce this issue, one can take https://github.com/dvojtise/mde-crashcourse-logo/tree/master/part3-mmfirst-solution projects and then modify the https://github.com/dvojtise/mde-crashcourse-logo/blob/master/part3-mmfirst-solution/fr.inria.sed.logo.xdsml/src/fr/inria/sed/logo/xdsml/ui/turtleboard/TurtleBoardEngineAddon.java method engineStarted by removing the `if(turtleGUI != null) {` test which is a workaround for this issue.