Toggles for extension menus

[gsteinLTU]

Extensions currently can define menus with submenus and dividers:

Snap--Build-Your-Own-Blocks/src/gui-ext.js

Line 468 in 3ce84a3

const menuFromDict = dict => {

However, Snap!'s menus also include checkboxes for settings, which would be really nice to have for adding settings to extensions without each extension needing to design custom dialogs to have even one toggle. The function adding them is here:

Snap--Build-Your-Own-Blocks/src/gui.js

Line 3342 in 3ce84a3

function addPreference(label, toggle, test, onHint, offHint, hide) {

To maintain backwards compatibility, it might make sense to add it as a getPreferences function to extensions which adds a settings submenu for the extension if any are specified.