[Backport IE11 fixes to 1.7]  Change focus manager to use dojo/on rather than addEventListener() and attachEvent() on older IE's.

cherry picked from https://github.com/dojo/dijit/commit/e2b992b720df9936af690e08de1a76c299f38613
+ manually backported the isFocusable function in dijit/a11y so the previous commit can be applied