Reimplement computation of change tracking status

Reimplement computation of change tracking status

This is a reimplementation of 6d4e6aad that is both simpler and more
complete.

This uses the updateBuffer mechanism to implement a fully working
version of Inset::isChanged(). Now the function returns true for an
inset that contains an inset that contains a change, for example.

Moverover Buffer::areChangesPresent() is merely a proxy for
Buffer::inset().isChanged().