Iframes work but the tab state isn't handled correctly

[fregante]

• The content script is injected in every document simultaneously but when selecting a field in an iframe, the outer frame keeps listening
• The fields in the iframe don't seem to be highlighted
• The "connected field count" in the browser action is managed separately by each content script, so they end up overriding each other
• alreadyInjected is only considered for the top-frame, but this should be handled for each frame (e.g. if you activate GT, then the iframe changes page, it will appear alreadyInjected but the iframe doesn't have any content scripts)

This is a rather complex issue and will probably require rewriting the injection logic once again.