Add cancel button to 'Do you want to save?' message box

[rowo360]

This PR fixes #1183 by adding a cancel button to the 'Do you want to save?' message box.

Overall the MessageBox looks like this now:

From a technial point of view I introduced a new enum MessageBoxResult representing the return values of a MessageBox.
This enum is intended to pursue the idea of the IMessageBox interface further: hiding the implementation details of the Windows Forms MessageBox to the caller.
So far a boolean return value was sufficient for all methods of the IMessageBox interface - however the new method YesNoCancel requires more than a boolean state - do I decided to use that enum.