this to show a "your thumbs/resizes are out of date" message whenever
we change the graphics rules. Tweak watermark module to add graphics
rules whenever we make a change, which triggers the graphics module to
add the permanent message.
Track a set of rules in Graphics_Rule_Model which specify how we turn
original images into thumbnails and resizes. There's one set of rules
that applies to every image in the Gallery.
Track the state of thumbs and resizes with a "dirty" bit. The new
graphics helper manages the rules and can rebuild the thumbs and
resizes for any images that are considered "dirty".
Introduce the concept of an "album cover" which is an item that an
album points to. We'll use that item as the source for the album's
thumbnail/resize.
Conflated with this change (sorry!) I also changed the Var table to
use module_name instead of module_id. This may be marginally less
efficient, but it's much easier to follow in the database.
