'set_theme()' should accept "feature list"

[alshan]

LetsPlot.set_theme(theme_bw()+flavor_solarized_dark())

ValueError: Wrong option type. Expected theme but was feature-list.

Or provide other way, perhaps:

LetsPlot.set_flavor(flavor_solarized_dark())

[alshan]

Related issue in LPK concerning default color palettes: https://github.com/JetBrains/lets-plot-kotlin/issues/213

[RYangazov]

Fixed via 0b34814ec93de81317e0ec99ab77164f3f9a5669