shiny: add window title support

Titles are UTF-8 compatible on all platforms; drivers
default to an empty string.

Change-Id: I4cf1a16ceb4e9641b2e1fc78dcff0501aac7dedb
Reviewed-on: https://go-review.googlesource.com/37200
Reviewed-by: Nigel Tao <nigeltao@golang.org>
7 files changed