setFailedToLaunch has a weird asymmetric api: now launchErrorMessage is a string property.
failedToLaunch remains and is internally set true the first time a message is set (is not posssible to set it back to false)
the message is in turn used by the scriptengine for the user visible error message.
It is done in 2 places:
if the applet package is invalid (applet not found) the message is created by the containment
if the applet package is valid, but there is a parse error in the qml file the error message is generated by the scriptengine of the applet itself.
I'm still really not happy about how the error ui is generated.
AppletInterface is a QQuickItem ans
containments will receive pointers to AppletInterface as applets,
so will be able to have a limited access to it.
as an example of it working the button in the example applet enables/disables background hints
I'm still not 100% happy about it, we should see exactly what it happens when the containment deletes the graphic object.
