Simplify theming

[Swader]

Currently, switching out CSS / JS files is just a matter of replacing the assets.ini file with one configured for a different front end framework. However, the view logic makes things a bit more complicated. Due to each front end framework using its own syntax and classes, to change a theme one would have to go through all the up until now defined elements in the twig view files and change all classes and nesting to match the new theme. This is impractical to say the least.

One way to make this simpler (and to basically allow for the existence of a "theme marketplace") is to support "theme folders" in the Twig default view folders, and then nest the individual views under those, like they're nested now. So just add another level for the theme, and then reference the theme's folder in each render call by variable.