LayoutItemType.getLayoutID() should by LayoutItemType.getViewType()

[MarcusWolschon]

LayoutItemType
must implement a "fun getLayoutID() : Int".

a)
Most of the time, it is not used as a layout ID but just as a convenient value for itemViewType
b)
Hardcoding layout IDs prevents the use of ViewBinding to instantiate layouts.

Instead we could give the TreeViewBinder a function to instantiate a new view
with a default implementation that inflates a layout.

This way the provideViewHolder() method can be given a view of a specific type and thus increase type safety and null safety.