Draft TextInput widget structure

Also started a `todos` example to showcase it!

core/src/widget/text_input.rs

@@ -0,0 +1,84 @@
+use crate::Length;
+
+pub struct TextInput<'a, Message> {
+    pub state: &'a mut State,
+    pub placeholder: String,
+    pub value: String,
+    pub width: Length,
+    pub max_width: Length,
+    pub padding: u16,
+    pub size: Option<u16>,
+    pub on_change: Box<dyn Fn(String) -> Message>,
+    pub on_submit: Option<Message>,
+}
+
+#[derive(Debug, Default)]
+pub struct State {}
+
+impl<'a, Message> TextInput<'a, Message> {
+    pub fn new<F>(
+        state: &'a mut State,
+        placeholder: &str,
+        value: &str,
+        on_change: F,
+    ) -> Self
+    where
+        F: 'static + Fn(String) -> Message,
+    {
+        Self {
+            state,
+            placeholder: String::from(placeholder),
+            value: String::from(value),
+            width: Length::Fill,
+            max_width: Length::Shrink,
+            padding: 0,
+            size: None,
+            on_change: Box::new(on_change),
+            on_submit: None,
+        }
+    }
+
+    /// Sets the width of the [`TextInput`].
+    ///
+    /// [`TextInput`]: struct.TextInput.html
+    pub fn width(mut self, width: Length) -> Self {
+        self.width = width;
+        self
+    }
+
+    /// Sets the maximum width of the [`TextInput`].
+    ///
+    /// [`TextInput`]: struct.TextInput.html
+    pub fn max_width(mut self, max_width: Length) -> Self {
+        self.max_width = max_width;
+        self
+    }
+
+    /// Sets the padding of the [`TextInput`].
+    ///
+    /// [`TextInput`]: struct.TextInput.html
+    pub fn padding(mut self, units: u16) -> Self {
+        self.padding = units;
+        self
+    }
+
+    pub fn size(mut self, size: u16) -> Self {
+        self.size = Some(size);
+        self
+    }
+
+    pub fn on_submit(mut self, message: Message) -> Self {
+        self.on_submit = Some(message);
+        self
+    }
+}
+
+impl<'a, Message> std::fmt::Debug for TextInput<'a, Message>
+where
+    Message: std::fmt::Debug,
+{
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        // TODO: Complete once stabilized
+        f.debug_struct("TextInput").finish()
+    }
+}