Add arrow key controls to emoji and reaction pickers (#10637)

* Add arrow key controls to emoji and reaction pickers

* Iterate types

* Switch to using aria-activedescendant

* Add tests

* Fix tests

* Iterate

* Update test

* Tweak header keyboard navigation behaviour

* Also handle scrolling on left/right arrow keys

* Iterate

cypress/e2e/threads/threads.spec.ts

@@ -174,7 +174,7 @@ describe("Threads", () => {
             .click({ force: true }); // Cypress has no ability to hover
         cy.get(".mx_EmojiPicker").within(() => {
             cy.get('input[type="text"]').type("wave");
-            cy.contains('[role="menuitem"]', "👋").click();
+            cy.contains('[role="gridcell"]', "👋").click();
         });
 
         cy.get(".mx_ThreadView").within(() => {