[Bug 862310] Add network progress indication to apps/smart folder [r=crdlc]

Basically, for every apps API request (search / smartfolder) after a certain time (800ms) an indicator is shown in the center of the apps container until the response arrives.

![2013-04-16-15-47-07](https://f.cloud.github.com/assets/486954/385867/c9a22d1a-a694-11e2-8fbc-1805c819de09.png)