This is the current design:
This can be achieved in different ways. I suggest you create an EditorWidget (H5P.ShapeEditor) that only contains CSS. This shall be included in H5P.Shape's editorDependencies (library.json). You don't have to do anything more in H5P.Shape.
Make sure all the css in the editor widget is using .h5p-editor-shape in all selectors.
All the icons should be part of the EditorWidget, and should be possible to set on .h5peditor-label:before, using background-image.
A nicer editor