e.g. `layout.template.layout.annotationdefaults`

…properties

…essages

Supports registering/unregistering templates and setting default template

layout.template can now be specified as the name of a template, and
if layout.template is not specified, then a registered default is applied
during figure construction.

This inputs a figure and outputs a new figure where all eligible
properties have been moved into the new figure's template definition

By default properties named 'text' and 'title' are note moved into the
template, but this can be customized using the `skip` argument.

fig.layout.template = 'template1+template2'

and logic to lazily load theme from file on first use

going to be pretty large

decimate bmw colorscale in plotly templates to save space and reduce
plotting latency.

colorscale is used across trace types.

Also crop bwm colorscale slightly so that it has better contrast on both
light and dark themes. No it doesn't go quite as close to black or as
close to white.

of text and lines/markers for several trace types.

…in builds

There is still some non-determinism with raster image formats, and
eps is enough to execute the full orca pipeline

in the plotly_* templates

that can be added to turn it off if desired.

 - Slightly lighten background grid in plotly template
 - Slightly darken grid lines in plotly_dark template
 - Don't explicitly enable xgrid/ygrid in plotly* templates, let
   plotly.js decide.

This way fig.layout.template won't be None after Figure is constructed

…* templates

This makes zero lines less distracting when not helpful, while still
making it fairly easy to focus on them when desired.