Improving integration of Plotly.js templates (themes)

[jonmmease]

Some notes on integrating the new template (theme) support from plotly/plotly.js#2761 (original discussion from #924). With just rerunning code generation, in plotly.py 3.1+ template support is already usable. For example:

fig = go.FigureWidget()
fig.layout.template = {
    'layout': {'paper_bgcolor': 'lightgray'},
    'data': {'scatter': [{'mode': 'markers',
                          'marker': {'size': 20, 'symbol': 'square'}},
                         {'mode': 'markers',
                          'marker': {'size': 20, 'symbol': 'triangle'}},
                        ]}}
fig.add_scatter(y=[3, 1, 3])
fig.add_scatter(y=[1, 3, 2])
fig

Here are some of my thoughts for improving the integration for plotly.py users:

• Write a custom validator for layout.template (right now it accepts anything and Plotly.js just ignores invalid properties)
• Include some way to set a global template that is the default unless if it overridden in the constructor. Support saving this default to a ~/.plotly settings file.
• Include a set of predefined named templates (like @sglyon did in WIP: adding themes #924).
• Figure out what's going on with the new name and templateitemname properties that were added to all of the object array properties (e.g. annotations, images, etc.). I think the name properties should only show up in the template hierarchy and templateitemname should only show up in the normal object hiererahcy.

[jonmmease]

Done in #1224