Selections, masks, overlays

selections: 
 * read protection: only selected pixels can be read
 * write protection: only selected pixels can be written to 
 * read-write protection: only selected pixels can be read & written
to

masks:
  * associated with any filter
  * associated with the transparency filter

overlays
  * various painterly types



Selections are associated with KisLayer and KisPaintDevices
Masks are associated with KisLayer
Overlays are associated with KisPaintLayer

KisLayer has the masks and selections _and_ the final projection.
Layers also have an internal projection. KisLayer asks the layer for
the internal projection and then merges it with the effect masks into
the projection that is used in the final layer merge to deliver the
root layer projection.
