The idea here is to expose a set of "virtual widgets" that can be used with a
`Virtual` widget and its `virtual::State`.
A virtual widget is a widget that does not contain any state, but instead is a
"virtual" representation of the "real" widget. The real widgets are stored in
the `virtual::State`.
Every time a new virtual widget tree is created during `view`, it is compared to
the previous one and "real" widgets are added / removed to the `virtual::State`.
Effectively, this removes the need to keep track of local widget state in the
application state and allows `view` to take an immutable reference to `self`.
To summarize, using this crate should allow users to remove `State` structs
in their application state.
Eventually, the strategy used here may be adopted generally and, as a result,
all of the widgets in `iced_native` would be replaced!
