Remove GWT detection for now

There is no way of accessing iframe.contentWindow on 3rd party domains
without triggering a warning in the console. I'm not familiar with GWT,
so I'm not sure if this iframe method is a requirement or not. Since
this is blocking a release though, we can address this as a minor
version fix later.
Fix one more deprecation resulting from manifestv2

Use sendMessage instead of sendRequest
Use new background script option instead

This is simpler than the previous main.html method.
Increment version and update changelog