Fix "re-open ember panel on Firefox"

This commit fix a bug in the Firefox addon code:

if a user close and re-open the ember inspector on a target page, the ember inspector devtool panel does not detect the Ember App in the target page anymore.

This issue (related to lost messages due to a not yet connected worker) is resolved using postMessage API as message channel between Firefox add-on code and Ember Inspector DevTool panel 
(and this new approach simplifies firefox addon code as a nice side-effect)