Sets whether changes to the active layer should be temporarily
blocked. Exposes a previously private optimisation for use
by plugins.
This is a low-level method, designed to avoid unnecessary work when adding lots
of layers at once. Clients which will be adding many layers may call blockActiveLayerChanges( TRUE ) upfront,
add all the layers, and then follow up with a call to blockActiveLayerChanges( FALSE ). This will defer emitting
the active layer changed signal until they've added all layers, and only emit the signal once for
the final layer added.
