Corrected indentation, added license/copyright

Signed-off-by: Maximilian Güntner <maximilian.guentner@gmail.com>
Correct tabstops in MainTab

Signed-off-by: Maximilian Güntner <maximilian.guentner@gmail.com>
Fix the inconsistent behaviour of QCompleter

The TagWidget behaves now similiar to a QLineEdit.
Pressing Enter/Return will now close the completion widget.

Signed-off-by: Maximilian Güntner <maximilian.guentner@gmail.com>