Message “broadcasters” don’t have a single “other side” that they
send messages to, but rather a set of subordinate message managers.
For example, broadcasting a message through a window message
manager will broadcast the message to all frame message managers
within its window.
Like |sendAsyncMessage()|, but also broadcasts this message to
all “child” message managers of this message manager. See long
comment above for details.
WARNING: broadcasting messages can be very expensive and leak
sensitive data. Use with extreme caution.
Return a single subordinate message manager.
Number of subordinate message managers.