Code review comment for lp://staging/~alan-griffiths/miral/fix-1668651

I've considered maintaining an internal list of InternalClientRunners and calling join_client_thread() on all of them immediately *after* executing any client provided stop callbacks.

That would simplify the main() functions (miral-kiosk needs nothing, miral-shell just needs the hook to close the decorations provider).

It would also be possible to introduce a mechanism to notify internal clients of a close request by supplying overloads that provide a callback for this purpose.

But all that is providing convenience for simple usecases when the mechanisms provided here are needed to address the general case.