revert backend_thumb_height to 100

I'm not sure if I'm missing something here but the change of the default system setting for thumb height made in https://github.com/modxcms/Gallery/commit/1c3433192a5915da7ba46486953b7d84ca22fa8b from 100 to 80 just seem off to me. It just looks like the image did not load properly since the outer box is 100px. So I'll suggest a revert back to a hight of 100px.

See pic, manualy changed the url to get a 100px high image for the first one.
![003_selection](https://cloud.githubusercontent.com/assets/1887628/4346261/d63134f8-40f4-11e4-94d1-962d88f724bf.png)