Possible improvements with "context parameters" feature

KEEP: https://github.com/Kotlin/KEEP/issues/367
1) `LayerCreatorScope` should be reworked and named `DataScope` and provide only data-related logic.
* `plotBuilder` should not be member, temporary workaround until context parameters are ready
* `createLayer` should create and return Layer (not add it) and be used with `MultiLayerPlotBuilder.addLayer`
2) `DataFramePlotBuilder` can be improved as well (since in fact it's a combination of `DataFrame` and `PlotBuilder` contexts)

Start by reading KEEP issue 367 and tracing LayerCreatorScope, DataFramePlotBuilder, and MultiLayerPlotBuilder. Define the DataScope responsibilities, the createLayer and addLayer relationship, and how context parameters should simplify DataFramePlotBuilder. Done means the proposed API redesign is implemented consistently across these builders.

Written by the indexing model from the issue text.