[Save All] Show Save All option when closing window

When nested changes have been made in a window,
show the Save All option in the dialog that
pops up. Addresses remaining case of nasa/mct#105
[Save All] Avoid NPE when changing view

User input dialog may return null if the
user dismisses it without making a choice,
so avoid calling methods on the answer.
[Save All] Avoid NPE after dialog

User input dialog may return null (this
happens when user simply closes the dialog
instead of clicking a button); avoid
calling methods directly on the return
value of the input dialog, except where
null has been checked for first.
[Save All] Test dialog on housing close
Merge pull request #218 from VWoeltjen/mct105b

[Save All] Show 'Save All' option when closing window