set up demo, exercise, final folders

step1-06/exercise/src/components/TodoHeader.tsx

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+import React from 'react';
+
+export class TodoHeader extends React.Component<any, any> {
+  constructor(props) {
+    super(props);
+    this.state = { labelInput: '' };
+  }
+
+  render() {
+    const { filter } = this.props;
+
+    return (
+      <header>
+        <h1>todos</h1>
+        <div className="addTodo">
+          <input value={this.state.labelInput} onChange={this._onChange} className="textfield" placeholder="add todo" />
+          <button className="submit">Add</button>
+        </div>
+        <nav className="filter">
+          <button className={filter == 'all' ? 'active' : ''}>all</button>
+          <button className={filter == 'active' ? 'active' : ''}>active</button>
+          <button className={filter == 'completed' ? 'active' : ''}>completed</button>
+        </nav>
+      </header>
+    );
+  }
+  _onChange = evt => {
+    this.setState({ labelInput: evt.target.value });
+  };
+}