Splitting view logic into BaseViewAbleUIElement mixin

[CRImier]

Right now, BaseListUIElement contains a lot of view-related logic that other view-less UI elements could reuse - view config parsing, view selection with fallbacks and so on. We should split that logic into a mixin class, as well as provide a base class for views (with stuff like __init__ for a start, to be expanded later when view plugins become a thing). This should help us moving towards solving #3, too.

[CRImier]

Now in the ofono_bridge branch, implemented as BaseViewMixin + a BaseView class - UI elements that had views are already ported to it, now just needs some testing and documentation.