(core) Add behavioral and coaching call popups

Summary:
Adds a new category of popups that are shown dynamically when
certain parts of the UI are first rendered, and a free coaching
call popup that's shown to users on their site home page.

Test Plan: Browser tests.

Reviewers: jarek

Reviewed By: jarek

Differential Revision: https://phab.getgrist.com/D3706

app/common/ThemePrefs-ti.ts

@@ -75,6 +75,8 @@ export const ThemeColors = t.iface([], {
   "tooltip-close-button-fg": "string",
   "tooltip-close-button-hover-fg": "string",
   "tooltip-close-button-hover-bg": "string",
+  "tooltip-popup-header-fg": "string",
+  "tooltip-popup-header-bg": "string",
   "modal-bg": "string",
   "modal-backdrop": "string",
   "modal-border": "string",