Add step for opacity slider (#670)

Instead of freeform, let's only support 10 increments of opacity. Should help keep things consistent

src/actions/actionProperties.tsx

@@ -218,6 +218,7 @@ export const actionChangeOpacity: Action = {
         type="range"
         min="0"
         max="100"
+        step="10"
         onChange={e => updateData(+e.target.value)}
         value={
           getFormValue(