Add special validator support for layout title attributes for Plotly.js 1.43.0

[jonmmease]

In plotly.js 1.43.0 the layout.title attribute is now a container that contains sub-attributes like text, font, xanchor, etc. The old behavior of specifying layout.title as a string and using layout.titlefont as the font container is deprecated.

Plotly.py should at least support setting layout.title as a string and setting layout.titlefont but internally map these properties to the new version.

E.g. fig.layout.title = 'A Title' is internally converted to fig.layout.title.text = 'A Title'.

Similarly, fig.layout.titlefont.family = 'Courier' is internally converted to fig.layout.title.font.family = 'Courier'.

Looks like axis titles like fig.layout.xaxis.title have also been converted into containers.

In all cases, a DeprecationWarning should be raised.

[jonmmease]

Done in #1376