Bug 732787 - Posterize doesn't work as expected with alpha channel

GimpOperationPosterize: also posterize the alpha channel. Seems like a
no-brainer to me, and is actually useful (see bug). The old behavior
is still available by simply disabling the alpha channel in the
channels dialog.
app: allow to change the affected channels on a running filter

Connect GimpImageMap to the GimpImage's "component-active-changed" signal
and update the channels affected by the filter.