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/element/showSelectedShapeActions.ts

@@ -0,0 +1,13 @@
+import { AppState } from "../types";
+import { ExcalidrawElement } from "./types";
+import { getSelectedElements } from "../scene";
+
+export const showSelectedShapeActions = (
+  appState: AppState,
+  elements: readonly ExcalidrawElement[],
+) =>
+  Boolean(
+    appState.editingElement ||
+      getSelectedElements(elements).length ||
+      appState.elementType !== "selection",
+  );