CopyableLabel: Fix select all on Safari

By using a deferred command here for the select all and focus we
can better handle Safari's event model, which requires the UI to
update before select all will work.

Change-Id: Ie3a190f3567d3cb05beb3714d6e2b4b02ad520ac
Signed-off-by: Shawn O. Pearce <sop@google.com>