Renamed the UI project to avoid confusion

Renamed the UI project to avoid confusion, let's hope it will help to
avoid this kind of actions in the future, because the loss of diff could
hurt.
Added basic data model - preview

Added simple data model, that I've been working on this evening. It
contains task, category, priority and all appropriate containers, that I
would like to be single instanced in the app. I think it will enable
building the basic functionality with small adjustments.