title: Error using tooltip argument with python dash slider
body: <p>I get an error when I use the tooltip={'always_visible': True} argument when creating a dash slider in python.</p>
  
  <p>The error only appears when I move the slider.</p>
  
  <p>When I remove the tooltip argument, the code runs fine (but I don't get the tooltip that I need).</p>
  
  <pre class="lang-py prettyprint-override"><code>
  # Here's a minimal example that reproduces the error
  
  import dash
  import dash_html_components as html
  import dash_core_components as dcc
  
  app = dash.Dash(__name__) 
  app.layout = html.Div([
      dcc.Slider(
          id='my-slider',
          min=0,
          max=20,
          step=0.5,
          value=5,
          tooltip={'always_visible': True}
      ),
      html.Div(id='output-container-slider')
  ])
  
  @app.callback(
      dash.dependencies.Output('output-container-slider', 'children'),
      [dash.dependencies.Input('my-slider', 'value')])
  def update_output(value):
      return 'You have selected "{}"'.format(value)
  
  if __name__ == '__main__':
      app.run_server(debug=True)
  </code></pre>
  
  <p>I would expect the tooltip to show the current value of the slider as it moves and the tooltip to persist even I move the mouse away.</p>
  
  <p>Instead, I get this error:</p>
  
  <pre><code>Failed component prop type: Invalid component prop `tooltip` key `visible` supplied to Slider.
  Bad object: {
    "visible": true
  }
  Valid keys: [
    "always_visible",
    "placement"
  ]
  
  (This error originated from the built-in JavaScript code that runs Dash apps. Click to see the full stack trace or open your browser's console.)
  Error: Failed component prop type: Invalid component prop `tooltip` key `visible` supplied to Slider.
  
  Bad object: {
    "visible": true
  }
  
  Valid keys: [
    "always_visible",
    "placement"
  ]
  
      at propTypeErrorHandler (http://127.0.0.1:8050/_dash-component-suites/dash_renderer/dash_renderer.dev.js?v=1.0.1&amp;m=1568039086:44125:11)
  
      at CheckedComponent (http://127.0.0.1:8050/_dash-component-suites/dash_renderer/dash_renderer.dev.js?v=1.0.1&amp;m=1568039086:40498:9)
  
      at renderWithHooks (http://127.0.0.1:8050/_dash-component-suites/dash_renderer/react-dom@16.8.6.js?v=1.0.1&amp;m=1568039086:13073:18)
  
      at mountIndeterminateComponent (http://127.0.0.1:8050/_dash-component-suites/dash_renderer/react-dom@16.8.6.js?v=1.0.1&amp;m=1568039086:15155:13)
  
      at beginWork (http://127.0.0.1:8050/_dash-component-suites/dash_renderer/react-dom@16.8.6.js?v=1.0.1&amp;m=1568039086:15760:16)
  
      at performUnitOfWork (http://127.0.0.1:8050/_dash-component-suites/dash_renderer/react-dom@16.8.6.js?v=1.0.1&amp;m=1568039086:19447:12)
  
      at workLoop (http://127.0.0.1:8050/_dash-component-suites/dash_renderer/react-dom@16.8.6.js?v=1.0.1&amp;m=1568039086:19487:24)
  
      at renderRoot (http://127.0.0.1:8050/_dash-component-suites/dash_renderer/react-dom@16.8.6.js?v=1.0.1&amp;m=1568039086:19570:7)
  
      at performWorkOnRoot (http://127.0.0.1:8050/_dash-component-suites/dash_renderer/react-dom@16.8.6.js?v=1.0.1&amp;m=1568039086:20477:7)
  
      at performWork (http://127.0.0.1:8050/_dash-component-suites/dash_renderer/react-dom@16.8.6.js?v=1.0.1&amp;m=1568039086:20389:7)
  </code></pre>
  
  <p>Thanks for the help!</p>