Legend to export as image

[GitGoogol]

When the exported graph is used for documentation stuff, there is no information what the different colors and arrows mean.
Providing an optional legend by exporting the graph as image would be great.

[egraether]

Implemented with 2018.2.122
Show legend via 'legend' command or '?' button in the lower left corner of the graph. Then exportable as image as usual via context menu.