Extend layer legend symbol styles

[nesnoj]

Currently in createLegend / class LegendLayer only the background color is used for legend symbol of a layer.
For the visual match it'd be good to support:

• fill-outline-color as border
• fill-pattern as symbol
• Line symbol (already supported?)
• When using different colors (example see Support discrete values in choropleth legend #17) maybe use 2 stripes with first and last used color? (as in the egon app choropleth legend)