Introduce UI elements

[shikaan]

To build UI in the engine, we need some Objects that do not generally behave like other Objects in the game.

Currently we only have a very opinionated ViewObject which is serving only one case. Despite the name suggesting a different level of abstraction.

A better idea, would be to have a set of UI elements (e.g., ViewObjects) that can be used as basis and extended according to the game needs.

The bare minimum is:

• Text: maybe split in label/paragraph;
• Button: something you can focus and click with a label;
• Input: to allow inputting characters;
• Checkbox/Radio: to select something;