Allow TruncatedList to get children via a callback
And update MemberList to use it as such. This means that the parent
only needs to make react elements for the elements that will
actually be rendered, rather than all of them.
In practive this doesn't make a huge difference as making React
elements is fairly fast, but experimentally (with all profiling
turned on), MemberList went from 25ms in the constructor and
81ms in render to 38ms in constructor but sub 1ms render for
Matrix HQ.
