Calling `puter.ui.launchApp()` now treats the new app as a child of the
one that launched it.
A child app is given a `puter.parent_instance_id` URL param, containing
its parent's instance ID.
Previously, `launchApp()` would resolve as soon as the app was launched.
This commit changes that, so that it is notified after the child app
sends its READY event to Puter. This means that as soon as `launchApp()`
has completed, the child app is ready to receive messages. The downside
is that launching an app that does not include Puter.js will now not
cause a notification, so `await puter.ui.launchApp()` will not resolve
in that case.
This lets apps communicate with each other, via Puter.
We probably want to limit this in some way, but for now, all apps are
allowed to send messages to any other apps.
The message is:
- `targetAppInstanceID`: The instance ID to send the message to
- `targetAppOrigin`: targetOrigin passed to postMessage(), in case we
want to restrict which URL can receive the message
- `contents`: The message to send to the target
The resizing and repositioning is safe from abuse in that the window's position and size cannot cause it to escape the viewport. More could be done here, e.g. rate limit resize/repos. I will request rate-limiting in a separate issue.
