Add a CloseWithoutClear copy mode key assignment

[LeszekSwirski]

Closing the copy overlay currently unconditionally clears the viewport, in particular resetting scroll. For the search overlay, we don't necessarily want to scroll back to the prompt after finding a match -- indeed, the old search overlay (which didn't use copy mode) had this behaviour.

Add a CloseWithoutClear key assignment which has this desired behaviour, and make it the default for ESC in search mode.

[LeszekSwirski]

This fixes #3502

[LeszekSwirski]

Added a helper and dropped ResetViewport, PTAL (and let me know if you prefer me to squash the individual commits)

[LeszekSwirski]

Ping

[LeszekSwirski]

Ping again (@wez)

[LeszekSwirski]

@wez anything more to do here?

[LeszekSwirski]

Regular ping for review, @wez

[wez]

Thank you!