(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/ui/OnBoardingPopups.ts

@@ -326,7 +326,7 @@ const Container = styled('div', `
   align-self: center;
   border: 2px solid ${theme.accentBorder};
   border-radius: 3px;
-  z-index: 1000;
+  z-index: ${vars.onboardingPopupZIndex};
   max-width: 490px;
   position: relative;
   background-color: ${theme.popupBg};
@@ -423,7 +423,7 @@ const Overlay = styled('div', `
   height: 100%;
   top: 0;
   left: 0;
-  z-index: 999;
+  z-index: ${vars.onboardingBackdropZIndex};
   overflow-y: auto;
 `);