Should we have 'style source objects' (and more generally, source objects)?

[dhrogers]

@JonasLukasczyk : For plots, it would be nice to have something like a 'style source object' that could provide a stylesheet for a plot. The system could come with different style sheets, and they could be changed globally. Example:

1. User connects a table output to an input of a plot view
2. User creates a 'plot style source' filter, and attaches it as an input to the plot view.
3. The plot now uses that style.

Since it is a source object, it could be included in the pycinema module, and wouldn't have to be a file on disk.