As a component of allowing arbitrary CCs, we must allow for the Esc key
to be pressed to clear autocomplete suggestions. The default behavior of
gr-overlay is to close when Esc is pressed.
This change supplies a property to the reply dialog gr-overlay to
prevent automatically closing on Esc, and fires the cancel event
manually.
Feature: Issue 5832
Change-Id: Iff4d8ea49dace83bb76d0e678ffc548c101658d1