Allow arbitrary metadata fields to be displayed as overlays on the canvas

## Use Case

@antoineborensztejn would like to rapidly produce screenshots of different scenes which highlights one or more pieces of metadata that differ between scenes. This would be easy if he could easily choose to superimpose a text object containing that metadata field over the viewport.

## Solution

The user should be able to easily choose to superimpose a text object containing a metadata field over the viewport.

This could mean fully dragging and dropping from the metadata viewer into the viewport, or just stacking the fields in a corner automatically. Metadata fields might have a handle for dragging in the first case, or a checkbox that makes them visible in the viewport in the second case.

#309 may lay the technical foundation for this.