Gas change event out of scope

[Davide DB]

Trying to set a gas change at the start of the dive I'm able to set it
out of scope.
The event is saved but not recognized as such.
So I'm able to add a following gas switch to the same bottle and while
removing I get a wrong runtime in the confirmation dialog.

http://trac.subsurface-divelog.org/ticket/831

Screencast:

https://www.screenr.com/vTEN

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Davide
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