title: adding layout to tabs on bokeh dashboard
body: <p>I'm exploring the bokeh library. 
  I tried to add several plots to each tab using VBox, but it did not work.
  I read somewhere that tabs &amp; VBox/HBox cannot be used together.<br>
  How do I handle the layout on the tabs then? </p>
  
  <p>Modified example to add several elements per tab:</p>
  
  <pre><code>from bokeh.models.widgets import Panel, Tabs
  from bokeh.io import output_file, show
  from bokeh.plotting import figure
  from bokeh.models.widgets.layouts import VBox
  output_file("slider.html")
  p1 = figure(plot_width=300, plot_height=300)
  p1.circle([1, 2, 3, 4, 5], [6, 7, 2, 4, 5], size=20, color="navy", alpha=0.5)
  
  p2 = figure(plot_width=300, plot_height=300)
  p2.line([1, 2, 3, 4, 5], [6, 7, 2, 4, 5], line_width=3, color="navy", alpha=0.5)
  p=VBox(p1,p2)
  tab1 = Panel(child=p,title="circle")
  tab2 = Panel(child=p2, title="line")
  tabs = Tabs(tabs=[ tab1, tab2 ])
  show(tabs)
  </code></pre>
  
  <p>Example from the website:</p>
  
  <pre><code>from bokeh.models.widgets import Panel, Tabs
  from bokeh.io import output_file, show
  from bokeh.plotting import figure
  
  output_file("slider.html")
  
  p1 = figure(plot_width=300, plot_height=300)
  p1.circle([1, 2, 3, 4, 5], [6, 7, 2, 4, 5], size=20, color="navy", alpha=0.5)
  tab1 = Panel(child=p1, title="circle")
  
  p2 = figure(plot_width=300, plot_height=300)
  p2.line([1, 2, 3, 4, 5], [6, 7, 2, 4, 5], line_width=3, color="navy", alpha=0.5)
  tab2 = Panel(child=p2, title="line")
  
  tabs = Tabs(tabs=[ tab1, tab2 ])
  
  show(tabs)
  </code></pre>