Add an attach debugger dialog.

Bug 15759123

Change-Id: I363524a191d319e03ce2b874cfc853c7330b684f
Reviewed-on: https://swiftshader-review.googlesource.com/3735
Reviewed-by: Nicolas Capens <capn@google.com>
Tested-by: Nicolas Capens <capn@google.com>