We did a rethink on channels, selections and masks last summer.

Traditionally, a mask is seen as an extra channel for an image, analogous
to the alpha channel. We want to do away with that idea.

A layer can have a number of permanent selections associated with it.
Such a selection can handle a composite method (determining the composition method
for the selected pixels) or a filter (making it an adjustment layer). 

Of course, a selection is just an 8-bit per pixel single channel layer that
belongs to a layer, so all filters can work automatically on a selection,
except that we don't have the ui for that yet.
