Adds support for generated grid backgrounds.
Features
- Grid backgrounds now have defaults for fill/stroke written into the SVG.
- The background plugin no longer assumes a type of "simple" by default: you now have to always supply a type.
- Finalized support for image backgrounds in 5.x. This functionality had never been thoroughly ported from 2.x.
- Added support to the Orthogonal connector to ingest connector geometry written by a 2.x version of the Toolkit.
- Internally refactored background plugin to expose type members that can be used when configuring backgrounds in an ECMAScript 6/TS development environment, to avoid the code being omitted through tree shaking.
- Added support for generated grid backgrounds. The surface widget creates and positions an SVG element in the background onto which a grid is drawn. As the bounds of your content changes, the element is sized and positioned automatically.
