This resolves several problems with handling of the saved cursor when
switching between primary and alternate screen. Additionally ref-tests
are also added for all common interactions to make sure the behavior
does not regress.
The behavior is based on XTerm's behavior except for interaction with
`reset`. XTerm does not reset the alternate screen's saved cursor on
`reset`, but VTE does. Since a `reset` should reset as much as possible,
Alacritty copies VTE here instead of XTerm.
Christian Duerr