Remove css injection from svg-edit extensions and include in svg-editor.html head

- Minor svg-edit extensions updates
Added 'show' method to svg-edit prompt extension api + minor updates
Revamp of svg-edit snapshots extension

- Works with svg-edit 2.6, updated some functionality and cleaned up code
- Implement public api
- Visual and UI improvements
- A few minor changes to description and stamps extensions
Added icon for import student work extension to the updated svg-edit extensions directory