feat: add stateful widgets

[fdehau]

Most widgets can be drawn directly based on the input parameters. However, some
features may require some kind of associated state to be implemented.

For example, the List widget can highlight the item currently selected. This
can be translated in an offset, which is the number of elements to skip in
order to have the selected item within the viewport currently allocated to this
widget. The widget can therefore only provide the following behavior: whenever
the selected item is out of the viewport scroll to a predefined position (make
the selected item the last viewable item or the one in the middle).
Nonetheless, if the widget has access to the last computed offset then it can
implement a natural scrolling experience where the last offset is reused until
the selected item is out of the viewport.

To allow such behavior within the widgets, this commit introduces the following
changes:

• Add a StatefulWidget trait with an associated State type. Widgets that
  can take advantage of having a "memory" between two draw calls needs to
  implement this trait.
• Add a render_stateful_widget method on Frame where the associated
  state is given as a parameter.

The chosen approach is thus to let the developers manage their widgets' states
themselves as they are already responsible for the lifecycle of the wigets
(given that the crate exposes an immediate mode api).

The following changes were also introduced:

• Widget::render has been deleted. Developers should use Frame::render_widget
  instead.
• Widget::background has been deleted. Developers should use Buffer::set_background
  instead.
• SelectableList has been deleted. Developers can directly use List where
  SelectableList features have been back-ported.

[clevinson]

Awesome! This will also make the "reverse scrolling" behavior I implemented for the paragraph widget a lot easier to work with (I can now have doubly bounded scroll boundaries!). I'll try to refactor that using stateful widgets and PR if there's anything that seems useful.