shiny/screen: change EventQueue to be a deque.

This sets us up for event filters such as gesture recognition, which generate
new events based on the most recent event.

Change-Id: I1aca2e4b1efb6846cd620ffef5b41b32e8cf7dd6
Reviewed-on: https://go-review.googlesource.com/24160
Reviewed-by: David Crawshaw <crawshaw@golang.org>
6 files changed