Bug 588050 - Cache information icon forever, only look up recent once

Extend ShellTextureCache by adding the concept of a policy, which
we expose to the public API for loading URIs.

This lets us have the shell tell the cache to keep the information
icon texture around forever.

Secondly, fix the caching of recent info; we shouldn't always be
loading the backup pixbuf.  Move recent info loading entirely
into ShellTextureCache.

js/ui/docDisplay.js

@@ -58,7 +58,8 @@ DocDisplayItem.prototype = {
         if (this._docInfo.mimeType == null || this._docInfo.mimeType.indexOf("image/") != 0)
             return null;
 
-        return Shell.TextureCache.get_default().load_uri_sync(this._docInfo.uri, availableWidth, availableHeight);
+        return Shell.TextureCache.get_default().load_uri_sync(Shell.TextureCachePolicy.NONE,
+                                                              this._docInfo.uri, availableWidth, availableHeight);
     }
 };