fix(windows): represerve keys on setfocus

Fixes #4557.

When focus changes, we need to re-run `_PreserveAltKeys` as settings may
have changed, meaning we have to preserve a different set of keys. I
also made the `_PreserveAltKeys` function idempotent so that we don't
have to worry about cleanup before calling it -- by calling the cleanup
function internally instead.