suggestion: allow ids and data-attributes on nodes

[dirkpuis]

Currently the generated SVG nodes have no IDs. On big drawings that makes debugging harder. Keeping IDs in the shadow DOM and in the generated SVG would also allow a search mechanism in the shadow DOM (a kind of 'get shape by ID').
Would also be nice if we could store some user data on the shadow DOM that gets written to the SVG nodes as attributes (e.g. the user might want something like data-selected-node="true" on a node). Being able to search on this type of data in the shadow DOM would be a very powerful addition.