Graphite - Colour Gradients

I have implemented colour gradients for Graphite, a work in progress vector editor written in the Rust programming language with Web Assembly.  The colour gradient tool is a new tool based on the new overlays system by @Truedoctor that allow users to draw in gradients, edit the start and and points by dragging the handles and edit the colours by using the new properties panel on the right hand side which has been added by @mfish33. View the code on GitHub.


Challenges for this MR included mapping the graident start and end points correctly into gradient units as well as making it so all of the gradients went into one `<defs>` tag at the top of the svg.