Copy/pasting vector and raster content between other graphics apps

*This is a good first issue but not for beginners.*

We should be able to copy vector and raster content, including hierarchies of layers and groups, between creative programs. Both into Graphite and out of Graphite. This shouldn't break existing copy-pasting within Graphite, which needs to have actual graph information rather than just vector or bitmap layer info.

This will require research into how the clipboard APIs let us encode different types of data, and if perhaps we need a degraded experience on web due to API limitations compared to a full separate implementation for the Tuari desktop version. Testing other creative apps and using a utility for inspecting clipboard contents at a technical level will be a useful way to approach this. It's also worth checking if this differs between OSs.

Helpful reading: https://alexharri.com/blog/clipboard