Now the element / fg / bg functions are passed to the actual Layout
function, not first put to the style. This avoids moving of the
element function to heap.
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.