Hi,

I am trying to create a slider that has \mu_1 as a title. I tried using `title=r'\mu_1'`

, which just gave me the literal string not the actual greek symbol. I also tried using MathML, which appears to be working in graphs but throws an error when I try to use it in the title of a slider.

```
from bokeh.models import Slider, MathML
mu_1 = """
<math>
<mrow>
<msub>
<mi>&mu</mi>
<mn>1</mn>
</msub>
</mrow>
</math>
"""
my_slider = Slider(title=MathML(text=mu_1),
value=0.5,
start=-1.0,
end=1.0,
step=0.1)
```

This gives me the following error message

```
ValueError: failed to validate Slider(id='1113', ...).title: expected either None or a value of type String, got MathML(id='1112', ...)
```

Is there an easier way to do this? Do sliders and other UI elements support MathML?