Add dialog for create datasource from selection (#346)

This should take care of #346. Now when a user creates a datasource from a selection, a dialog is displayed in which he can write a new name. A suggested name is offered to the user. This dialog gives warning messages for an empty name ("") and if a datasource already exists with the given name. I included French translations for all the new text and made sure the same text was used in the MapEditor and the TableEditor.