Bug 1102171 - [Usage][Refactoring] ViewManager.loadPanel should cleanup panels DOM once scripts/links are activated. r=salva

IMO relying on selector with ```href``` or ```src``` is more robust than setting ```id``` (```id``` like ```\path\path_1\file.css``` looks a bit weird :) ), but please let me know your thoughts?

Also I believe speed of ```head > link``` selector should be comparable with one for ```document.getElementById```