(core) Polish tutorial popups

Summary:
Includes the following changes:
 *  Adds "Click to expand" hover tooltip to all images
 *  Adds support for minimize/maximize by double clicking tutorial popup header
 *  Add New menu (and all other popups) should now persist when user moves tutorial popup
 *  Preserves scrollbar position when minimizing and maximizing tutorial popup
 *  Formula cell editor (and other elements) should now be stacked under tutorial

Test Plan: Browser and manual tests.

Reviewers: jarek

Reviewed By: jarek

Differential Revision: https://phab.getgrist.com/D3864

app/client/ui2018/cssVars.ts

@@ -134,6 +134,18 @@ export const vars = {
   logoBg: new CustomProp('logo-bg', '#040404'),
   logoSize: new CustomProp('logo-size', '22px 22px'),
   toastBg: new CustomProp('toast-bg', '#040404'),
+
+  /* Z indexes */
+  menuZIndex: new CustomProp('menu-z-index', '999'),
+  modalZIndex: new CustomProp('modal-z-index', '999'),
+  onboardingBackdropZIndex: new CustomProp('onboarding-backdrop-z-index', '999'),
+  onboardingPopupZIndex: new CustomProp('onboarding-popup-z-index', '1000'),
+  floatingPopupZIndex: new CustomProp('floating-popup-z-index', '1002'),
+  tutorialModalZIndex: new CustomProp('tutorial-modal-z-index', '1003'),
+  notificationZIndex: new CustomProp('notification-z-index', '1100'),
+  browserCheckZIndex: new CustomProp('browser-check-z-index', '5000'),
+  tooltipZIndex: new CustomProp('tooltip-z-index', '5000'),
+  // TODO: Add properties for remaining hard-coded z-indexes.
 };
 
 /**