Cannot use custom svg path / icons

Whenever I provide a custom SVG tag string or use `require('./icon.svg')` my ToolBar just shows empty square instead of an icon.

What I tried:
- providing any SVG path from [this website](http://svgicons.sparkk.fr/) and using `width="19" height="13" viewBox="0 0 19 13" xmlns="http://www.w3.org/2000/svg"` settings
- creating simple SVG path myself using [this path builder](https://codepen.io/anthonydugois/pen/mewdyZ)
- using any simple letter path from [this pen](https://codepen.io/scnijland/pen/xRGjBw)
- using any SVG from FontAwesome icon pack

Of course, if I use the SVG path provided in your repos icons show up as expected.

Is there any reason for this? Do you have a list of icons I could use that would work? I'd like to have even something as simple as a letter instead of picture icon.

Thank you for making this editor! It's great :)

Start by reading the ToolBar icon handling and compare how repository SVG paths are supplied with the custom SVG tag and require('./icon.svg') examples. Determine why those inputs render as empty squares; done means a custom SVG or simple letter icon displays correctly, with the supported icon format documented.

Written by the indexing model from the issue text.