Plasma applets when they were all C++ had a way for them to signal when
they were ready for showing to the user.
The QtQuick plugin does it automatically when an applet is compiled and
the component is created with no scope for the applet to do something
This is initially intended just for the desktop wallpaper, but is
generic enough for any applet/containment that does async loading on
API is chosen as we need something ref-count based to handle wallpaper
and containment both loading something asynchronous, and limited by the
fact that AppletInterface is not exported.