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.
That'd sound like a wontfix then. The better "experience" would be if I installed Facebook on one Marketplace, the rest of the Marketplaces should figure out that Facebook is installed too.