App rewrite changes

Hey Andrea, this branch contains a few changes I made while merging the timeline into my little dance project app.  I tried to make sure the commits were separate enough that you can just cherry pick things as you want them.

* Small tweaks to build/run under the app_rewrite branch SHA: ecaed8a48a2927da35e604b3079028315709f3e7
* If the app doesn't have an asset directory around, don't crash.  SHA: 15be7afd7bf595b69628baf40b121a63f0924831
* Added start and stop to QTimelineModule.  I found it useful for my modules to know when they start and stop.  In bigger apps, it'd probably be good to free memory or release textures at stop. SHA:  9b3046660a6b855174b7b69bb5b40b77e6b555c9
* QTimelineTrack keeps a pointer to the currently active QTimelineItem in the track, I did this to support using the tracks to control render order in the sample app.  I think having the currently active timeline item will be useful for other things in the future. SHA: 09b7f847a943d0caeb71dfdc65afe3dd24276677 and SHA: 5d125418c345dafcf8494f0b31939cd9ceb5ee4d

Hope you find this stuff helpful!