Show message box if no objects have been selected in Project view

There are several use cases in UGENE in which a user is offered to select some object(s) from the Project view. It takes place in such actions as adding objects to a document, adding sequences to an MSA, adding an annotation table to a sequence view, etc. Currently, if a user selects documents instead of objects nothing happens. This may be confusing for new users. A message box must be shown up to indicate that no objects have been selected.