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/components/HintViewer.css

@@ -1,5 +1,4 @@
 .HintViewer {
-  background-color: rgba(255, 255, 255, 0.88);
   color: #868e96; /* OC: GRAY 6*/
   font-size: 0.8rem;
   left: 50%;
@@ -9,9 +8,16 @@
   transform: translateX(calc(-50% - 16px)); /* 16px is half of lock icon */
 }
 
+.HintViewer > span {
+  background-color: rgba(255, 255, 255, 0.88);
+  padding: 0.2rem 0.4rem;
+  border-radius: 3px;
+}
+
 @media (max-width: 600px), (max-height: 500px) and (max-width: 1000px) {
   .HintViewer {
     position: static;
+    transform: none;
     margin-top: 0.5rem;
     text-align: center;
   }