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/structures/FilePanel.js

@@ -126,11 +126,12 @@ const FilePanel = createReactClass({
                     tileShape="file_grid"
                     resizeNotifier={this.props.resizeNotifier}
                     empty={_t('There are no visible files in this room')}
+                    role="tabpanel"
                 />
             );
         } else {
             return (
-                <div className="mx_FilePanel">
+                <div className="mx_FilePanel" role="tabpanel">
                     <Loader />
                 </div>
             );