Undo/Redo buttons, refactor menu toggles (#793)

* Make Undo & Redo and the menu buttons into actions; add undo/redo buttons

* Create variables for the ToolIcon colors

* Darken the menu buttons when they’re active

* Put the more intensive test in `perform`

* Fix & restyle hint viewer

* Add pinch zoom for macOS Safari

* Chrome/Firefox trackpad pinch zoom

* openedMenu → openMenu

* needsShapeEditor.ts → showSelectedShapeActions.ts

* Call showSelectedShapeActions

src/locales/en.json

@@ -60,7 +60,9 @@
     "resetZoom": "Reset zoom",
     "menu": "Menu",
     "done": "Done",
-    "edit": "Edit"
+    "edit": "Edit",
+    "undo": "Undo",
+    "redo": "Redo"
   },
   "alerts": {
     "clearReset": "This will clear the whole canvas. Are you sure?",