BS-11314 Add Unit test on operation validation. Case of operation of
FileWidget downloadable only.
BS-11314 Add unit test on FileWidget Migration
- check assignment type is set to operator when leftOperand in empty
- check referencedElements is set when leftOperand is a Document
Conflicts:
	bundles/plugins/org.bonitasoft.studio.migration/src/org/bonitasoft/studio/migration/custom/migration/form/FileWidgetDocumentMigration.java
BS-11314 Fix Validation and migration
- Validation is not done on an FileWidgetAction if the "download only"
option is true
- Migration on FileWidget operation completed : a referenced document in
leftOperand missed its referencedElement property.
Conflicts:
	bundles/plugins/org.bonitasoft.studio.migration/src/org/bonitasoft/studio/migration/custom/migration/form/FileWidgetDocumentMigration.java