Suggest user to initialize repository on drag-and-drop folder without repository

Currently, we can drag-and-drop a repository folder directly under any group in SmartGit, but if the repository doesn't exist, it just says "The dropped repository is no Git repository". So we have to do Repository > Add or Create... each time.

Instead, if the folder has no repository yet, the app could offer the user to initialize one, then add the repository to the group it was dragged to on the go.