install
Attaches component to host, and returns the action that detaches it again.
In a host that holds many children, the returned action must detach the component by identity. Positions shift as siblings come and go, so an index captured now can point at another child later.
Return
the action that detaches component and frees the region
Parameters
host
the component that holds the region being filled
component
the child to attach
index
the position among the host's slot children, always 0 for a region that holds one child