Implement `qr_code` example

examples/qr_code/Cargo.toml

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+[package]
+name = "qr_code"
+version = "0.1.0"
+authors = ["Héctor Ramón Jiménez <hector0193@gmail.com>"]
+edition = "2018"
+publish = false
+
+[dependencies]
+iced = { path = "../..", features = ["qr_code"] }

examples/qr_code/README.md

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+## QR Code Generator
+
+A basic QR code generator that showcases the `QRCode` widget.
+
+The __[`main`]__ file contains all the code of the example.
+
+<div align="center">
+  <a href="https://gfycat.com/heavyexhaustedaracari">
+    <img src="https://thumbs.gfycat.com/HeavyExhaustedAracari-size_restricted.gif">
+  </a>
+</div>
+
+You can run it with `cargo run`:
+```
+cargo run --package qr_code
+```
+
+[`main`]: src/main.rs

examples/qr_code/src/main.rs

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+use iced::qr_code::{self, QRCode};
+use iced::text_input::{self, TextInput};
+use iced::{
+    Align, Column, Container, Element, Length, Sandbox, Settings, Text,
+};
+
+pub fn main() -> iced::Result {
+    QRGenerator::run(Settings::default())
+}
+
+#[derive(Default)]
+struct QRGenerator {
+    data: String,
+    input: text_input::State,
+    qr_code: Option<qr_code::State>,
+}
+
+#[derive(Debug, Clone)]
+enum Message {
+    DataChanged(String),
+}
+
+impl Sandbox for QRGenerator {
+    type Message = Message;
+
+    fn new() -> Self {
+        QRGenerator {
+            qr_code: qr_code::State::new("").ok(),
+            ..Self::default()
+        }
+    }
+
+    fn title(&self) -> String {
+        String::from("QR Code Generator - Iced")
+    }
+
+    fn update(&mut self, message: Message) {
+        match message {
+            Message::DataChanged(mut data) => {
+                data.truncate(100);
+
+                self.qr_code = qr_code::State::new(&data).ok();
+                self.data = data;
+            }
+        }
+    }
+
+    fn view(&mut self) -> Element<Message> {
+        let title = Text::new("QR Code Generator")
+            .size(70)
+            .color([0.5, 0.5, 0.5]);
+
+        let input = TextInput::new(
+            &mut self.input,
+            "Type the data of your QR code here...",
+            &self.data,
+            Message::DataChanged,
+        )
+        .size(30)
+        .padding(15);
+
+        let mut content = Column::new()
+            .width(Length::Units(700))
+            .spacing(20)
+            .align_items(Align::Center)
+            .push(title)
+            .push(input);
+
+        if let Some(qr_code) = self.qr_code.as_mut() {
+            content = content.push(QRCode::new(qr_code).cell_size(10));
+        }
+
+        Container::new(content)
+            .width(Length::Fill)
+            .height(Length::Fill)
+            .padding(20)
+            .center_x()
+            .center_y()
+            .into()
+    }
+}