BS-7463
Improve validation mechanism.
Fix has been apply for Table too.

Conflicts:
	bundles/plugins/org.bonitasoft.studio.expression.editor/messages.properties
	bundles/plugins/org.bonitasoft.studio.expression.editor/src/org/bonitasoft/studio/expression/editor/i18n/Messages.java
	bundles/plugins/org.bonitasoft.studio.expression.editor/src/org/bonitasoft/studio/expression/editor/viewer/ExpressionCollectionViewer.java