Added feature to prevent users from sending empty messages

As mentioned in [#60](https://github.com/nurupo/ProjectTox-Qt-GUI/pull/60), GUI freezes upon pressing either `Enter` or `Return` key for a long time. This feature prevents user from sending blank messages. Well the `.pro` file which was added automatically along with this commit is awful. It was automatically added. I cloned again and made changes and pushed again and it was again automatically added to my pull request.