add support for Ext css prefix used by namespaced builds

[nsams]

This change is 

[bmoeskau]

Thanks for the PR. I can't merge it at the moment, since even though it only impacts the styles, it could be considered a "breaking" change if someone has customized their own styles using those CSS classes. Also, a few of the classes are actually specific to Extensible, so not sure that it makes sense to prefix those with Ext's base prefix. It would probably make sense to introduce an Extensible-specific prefix in code for Extensible styles, and I do have a task on my list to fully refactor the stylesheets (and convert to SASS) but not until 2.0. Anyway, I'll keep this PR open and come back to it later. Thanks.

[nsams]

do you have a roadmap/plan for 2.0?

[bmoeskau]

I have an internal road map that is subject to change, but I haven't yet announced anything publicly.