Since we have 100% control over what data gets embedded, there is no
reason to embed anything that doesn't pass the strict yaml parsing
and it's better we throw a panic right away so it's easy to catch
this during development.
The Model was getting unmaintanable mess. This is an attempt to refactor/rewrite the Model so that data of certain type is exposed in standardized way, offering certain standard manipulations for that data type, and on the GUI side, certain standard widgets to tied to that data.
This rewrite closes#72, #106 and #120.