use apps iframe for installs/launch (bug 1029687)

Use the iframe to:

- install apps
- query for installed apps
- launch apps

In market_button.html, check if the app is installed to render the right text rather than asynchronously updating the button after the app is rendered.
helpers_fireplace sounds good. Even better would be to have one inherit from the other like with do with settings and settings_local. I'll keep it in mind.