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_local.js` is that the correct name for this? I don't know how things are structured now. `settings_local.js` doesn't get checked in. `helpers_local.js` does?