Simplify cilantro/ui directory structure, merge models

The views/, regions/ and layouts/ sub-directories have been
removed. All modules that were contained in views/ are now in
ui/. View classes are now directly attached to `cilantro.ui`
rather than `cilantro.ui.views`.

The serrano/ modules have been merged into the cilantro/models/
modules for simplicity, but also because there were a few
cross-dependencies to cilantro's core modules.