[LMS-1257] introduced active callback counter

.../client/web/client/application/AbstractAsyncCallback.java

...lient/web/client/application/AsyncCallbackCollection.java

...lient/web/client/application/CallbackListenerAdapter.java

...eric/client/web/client/application/ICallbackListener.java

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

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

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

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

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

.../property_type/EntityTypePropertyTypeSelectionWidget.java

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

...cation/ui/search/DetailedSearchFieldsSelectionWidget.java

...ric/client/web/client/application/AuthenticationTest.java

.../client/web/client/testframework/AbstractGWTTestCase.java

...generic/client/web/client/testframework/ITestCommand.java

...eneric/client/web/client/testframework/RemoteConsole.java

...n/experiment/GenericExperimentAttachmentDownloadTest.java