Replace stateful widgets with the new iced_pure API

[hecrj]

This PR replaces the current stateful widgets with the new widget API introduced by iced_pure in #1284.

The changes here remove a lot of code and prepare the codebase for the soon-to-land widget operations described in iced-rs/rfcs#7.

Some of the most relevant changes are:

• iced_pure code has been removed and the crate is deprecated.

• The pure feature has been removed.

• Widget types are no longer re-exported in the root iced crate. Instead, they have to be properly imported with the full path (eg. use iced::widget::Text).

• The widget function helpers in iced_pure have been moved to iced::widget.

• A couple of new macros have been added in iced::widget: column and row. They are analogous to vec!. Therefore, the old approach

  use iced::widget::Column;
  
  Column::new().push("I am a column").push("with 2 children");

  can now be written as follows:

  use iced::widget::column;
  
  column!["I am a column", "with 2 children!"]

• The row and column function helpers are also available, and they are equivalent to Row::with_children and Column::with_children. These are useful when you have an iterator or a Vec of elements!

• Text::new now can take any ToString instead of Into<String>.

• All of the internal State types are no longer exported in iced.

Thanks to all of these changes, all of the examples are now much simpler and less verbose! For instance, here is the new counter example:

use iced::widget::{button, column, text};
use iced::{Alignment, Element, Sandbox, Settings};

pub fn main() -> iced::Result {
    Counter::run(Settings::default())
}

struct Counter {
    value: i32,
}

#[derive(Debug, Clone, Copy)]
enum Message {
    IncrementPressed,
    DecrementPressed,
}

impl Sandbox for Counter {
    type Message = Message;

    fn new() -> Self {
        Self { value: 0 }
    }

    fn title(&self) -> String {
        String::from("Counter - Iced")
    }

    fn update(&mut self, message: Message) {
        match message {
            Message::IncrementPressed => {
                self.value += 1;
            }
            Message::DecrementPressed => {
                self.value -= 1;
            }
        }
    }

    fn view(&self) -> Element<Message> {
        column![
            button("Increment").on_press(Message::IncrementPressed),
            text(self.value).size(50),
            button("Decrement").on_press(Message::DecrementPressed)
        ]
        .padding(20)
        .align_items(Alignment::Center)
        .into()
    }
}

Once the widget operations land soon, this API will satisfy all of the use cases supported by the stateful one (and more!), while being way less verbose and cumbersome.

[artursapek]

dat delta