message: Implement `Default` for `pure::State`
  Impure Applications using pure widgets can now use a struct implementing `Default`
diff: @@ -41,6 +41,14 @@ pub struct State {
  state_tree: widget::Tree,
  }
  +impl Default for State {
  + fn default() -> Self {
  + Self {
  + state_tree: widget::Tree::empty(),
  + }
  + }
  +}
  +
  impl State {
  pub fn new() -> Self {
  Self {
  