Add support for line breaks in Label

The code below does not render the \n as a new line (Bokeh 0.12.13)

```python
from bokeh.plotting import figure, output_file, show
from bokeh.models import  Label

output_file("test.html")
fig = figure()
fig.line(x=range(10), y=range(10))
label = Label(x=2, y=8, text="New\nLine")
fig.add_layout(label)
show(fig)
```
A similar issue (#994) was solved by #6879.