(core) Add info and hover tooltips

Summary:
Adds tooltip buttons to various parts of the UI that either open a popup with
information when clicked, or show a label on hover.

Test Plan: Project tests.

Reviewers: jarek

Reviewed By: jarek

Differential Revision: https://phab.getgrist.com/D3657

app/client/declarations.d.ts

@@ -113,7 +113,9 @@ declare module "app/client/components/commands" {
     public desc: string;
     public humanKeys: string[];
     public keys: string[];
-    public run: () => any;
+    public getDesc(): string;
+    public getKeysDesc(): string;
+    public run(): any;
   }
 
   export type CommandsGroup = any;