Make a message dialog for cancelling VE

Confirm dialog was replaced in refactor, so until this is merged the 'Cancel'
button is broken.

Bug: 68068
Change-Id: I77d1c82bf0d68013eef361174a134860a197cd44
Don't allow document save if we're already tearing down the surface

Otherwise you get "Uncaught TypeError: Cannot read property 'messages' of null"
if you press the save button late enough in the saving process. The bug refers
to a different error which I have not been able to reproduce.

Bug: 68073
Change-Id: Ia8fa325f1450329b6e0e4ee9af5302aa4857d637
Merge "Don't allow document save if we're already tearing down the surface"
Merge "Make a message dialog for cancelling VE"