Exit package creator: improve dialogue adding Save option

[cpina]

See the steps to see the dialogue in #2

The dialogue current has two options: Ok and Cancel.

It should probably have the standard options for exiting an application with unsaved changes. For example, LibreOffice says:
"""
Save changes to document "$NAME_OF_THE_DOCUMENT" before closing?

Your changes will be lost if you don't save them.

[Don't Save] [Cancel] [Save]
"""

Note that adding a "Save" button might need to deal with "Save, when saved exit" and deal what happens if the "Save" operation fails (no internet in this moment).

Probably #2 needs to be done before this ticket.