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.
I know, just asking. I don't like it because it makes the code ugly, and these spaces aren't being being sent over the wire.