TIghter threading behavior for Handlers

They are now syncExec, and all but one synchronize on an exterior variable.
Indeed, event queues make them fire often :
- CompilerLaunchError : obviously
- ClasspathError : the Xsource modification triggers a classpath Check through the listener added to projectSpecificSetting.

TODO : instead of this, In ScalaProject's projectSpecificStorage, fire a java model Delta
	that can be caught by ScalaPlugin's elementchanged.