Various ARIA a11y fixes.

Notate RightPanel tabs.
Shorten Screen Reader queues.
Make AccessibleTooltipButton screen reader friendly
Flatten DOM for Sticker button using React Fragments

Signed-off-by: Michael Telatynski <7t3chguy@gmail.com>

src/components/views/right_panel/HeaderButtons.js

@@ -91,7 +91,7 @@ export default class HeaderButtons extends React.Component {
 
     render() {
         // inline style as this will be swapped around in future commits
-        return <div className="mx_HeaderButtons">
+        return <div className="mx_HeaderButtons" role="tablist">
             { this.renderButtons() }
         </div>;
     }