move DropDownList to widget package

...b/client/application/SearchableEntitySelectionWidget.java

...lient/web/client/application/ui/GroupSelectionWidget.java

...plication/ui/data/DataSetSearchFieldsSelectionWidget.java

...lient/application/ui/data/DataSetTypeSelectionWidget.java

...lication/ui/experiment/ExperimentTypeSelectionWidget.java

...ent/application/ui/experiment/ProjectSelectionWidget.java

...t/application/ui/field/VocabularyTermSelectionWidget.java

.../application/ui/material/MaterialTypeSelectionWidget.java

...application/ui/property_type/DataTypeSelectionWidget.java

...lication/ui/property_type/PropertyTypeAssignmentForm.java

...ication/ui/property_type/PropertyTypeSelectionWidget.java

...ient/application/ui/sample/SampleTypeSelectionWidget.java

...client/web/client/application/ui/widget/DropDownList.java

...client/application/ui/widget/EntityRegistrationPanel.java