Added the concept of "permanent" messages that we show to admins. Use

this to show a "your thumbs/resizes are out of date" message whenever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.

modules/watermark/views/admin_watermarks.html.php

@@ -12,7 +12,7 @@
   <? else: ?>
   <h2> <?= _("Active Watermark") ?> </h2>
   <p>
-    <?= _("Note that changing this watermark will rebuild all of your thumbnails and resized images.") ?>
+    <?= _("Note that changing this watermark will require you to rebuild all of your thumbnails and resized images.") ?>
   </p>
   <p>
     <div class="image">