Fix return when user select Abort in multi button dialog

closes #166 (closed)

• Modify multi button dialog so that it has the option to have or not have button Abort. When there is Abort button, the return when user select it is -1. Option to return 0 when Abort is selected is removed to not be confused with selecting the first button which has index 0 too.
• For the case there are more than one stations, at the end of the loading, user are forced to select one of the station. The Abort button is removed to avoid confusion.