Changeset 6033

#1233

• created baseclass for jobs (ItemJob) which derives OperatorJobs and EngineJobs
• created special view for OptimizerJobs which derives from a more general view
• removed logic from domain class HiveExperiment and moved it into RefreshableHiveExperiment
• improved ItemTreeView
• corrected plugin dependencies
• fixed bug in database trigger when deleting HiveExperiments
• added delete cascade for Plugin and PluginData
• lots of fixes