better scala container addition tagging
Basic status handler for the Xsource flag
Switch ScalaPresentationCompilerProxy.failedCompilerInitialization to Status Handlers
Centralizing thread Management for handlers
Aggregate Initialization Failures in ScalaCompiler reference validation
Switch UIErrorReporter to status handlers
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.
"Desired source level" toggle

- creation
- modification of the Xsource flag on modification

- classpath error status handler monothreaded again

- Toggle sourceLevel in case of manual-only Xsource edition

- Grey out souceLevel if no useProjectSettings

- Prevent double dialog on close/clean project
Merge pull request #696 from huitseeker/topic/ScalaDesiredVersion

Scala Source Level + Xsource UI support and validation