[TODO] theming - Githubissues

[x] Add types Theme and SerializedDocument.
[x] Change most functions which take a Document to take a SerializedDocument instead, in particular the backend_display functions.
[x] Add themes to Document and Serializer and apply the theme during serialization.
[x] Add backend_theme to allow the backend to specify a theme, so the Blink backend can set sizing_mode="stretch_both".
[x] Add functions for reading/writing themes to/from disk.
[ ] Add functions for modifying themes more easily. WONT DO: Themes are considered immutable at the moment.
[x] Either reintroduce the Figure type, or consider Figure and Plot to be the same when theming as a special case.
[x] Traverse the full MRO when applying themes.
[x] Document this.

cjdoris / Bokeh.jl