Use an icon instead of text for the codeButton label in the toolbar

# User story
As an Anki user, 
when I use the note editor,
I want the codeButton to use a graphical icon rather than text label
so that the size and alignment is consistent across different Anki versions

# Additional information
## Related issues
- [Issue #2](https://github.com/mschon/anki-addon-editor-codeButton/issues/2)